diff --git a/.travis.yml b/.travis.yml
index 478e31c4398..bfc07e2b510 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,12 +7,18 @@ node_js:
 
 sudo: false
 
+env:
+  - workerCount=3
+
 matrix:
   fast_finish: true
   include:
   - os: osx
     node_js: stable
     osx_image: xcode7.3
+    env: workerCount=2
+  allow_failures:
+  - os: osx
 
 branches:
   only:
@@ -28,3 +34,6 @@ install:
 cache:
   directories:
     - node_modules
+
+git:
+  depth: 1
diff --git a/Gulpfile.ts b/Gulpfile.ts
index 6c91ef52cbb..2718669b1f7 100644
--- a/Gulpfile.ts
+++ b/Gulpfile.ts
@@ -34,7 +34,7 @@ import through2 = require("through2");
 import merge2 = require("merge2");
 import intoStream = require("into-stream");
 import * as os from "os";
-import Linter = require("tslint");
+import fold = require("travis-fold");
 const gulp = helpMaker(originalGulp);
 const mochaParallel = require("./scripts/mocha-parallel.js");
 const {runTestsInParallel} = mochaParallel;
@@ -449,7 +449,7 @@ gulp.task(tsserverLibraryFile, false, [servicesFile], (done) => {
 });
 
 gulp.task("lssl", "Builds language service server library", [tsserverLibraryFile]);
-gulp.task("local", "Builds the full compiler and services", [builtLocalCompiler, servicesFile, serverFile, builtGeneratedDiagnosticMessagesJSON]);
+gulp.task("local", "Builds the full compiler and services", [builtLocalCompiler, servicesFile, serverFile, builtGeneratedDiagnosticMessagesJSON, tsserverLibraryFile]);
 gulp.task("tsc", "Builds only the compiler", [builtLocalCompiler]);
 
 
@@ -503,7 +503,7 @@ gulp.task("VerifyLKG", false, [], () => {
     return gulp.src(expectedFiles).pipe(gulp.dest(LKGDirectory));
 });
 
-gulp.task("LKGInternal", false, ["lib", "local", "lssl"]);
+gulp.task("LKGInternal", false, ["lib", "local"]);
 
 gulp.task("LKG", "Makes a new LKG out of the built js files", ["clean", "dontUseDebugMode"], () => {
     return runSequence("LKGInternal", "VerifyLKG");
@@ -928,26 +928,6 @@ gulp.task("build-rules", "Compiles tslint rules to js", () => {
         .pipe(gulp.dest(dest));
 });
 
-function getLinterOptions() {
-    return {
-        configuration: require("./tslint.json"),
-        formatter: "prose",
-        formattersDirectory: undefined,
-        rulesDirectory: "built/local/tslint"
-    };
-}
-
-function lintFileContents(options, path, contents) {
-    const ll = new Linter(path, contents, options);
-    console.log("Linting '" + path + "'.");
-    return ll.lint();
-}
-
-function lintFile(options, path) {
-    const contents = fs.readFileSync(path, "utf8");
-    return lintFileContents(options, path, contents);
-}
-
 const lintTargets = [
     "Gulpfile.ts",
     "src/compiler/**/*.ts",
@@ -959,27 +939,72 @@ const lintTargets = [
     "tests/*.ts", "tests/webhost/*.ts" // Note: does *not* descend recursively
 ];
 
+function sendNextFile(files: {path: string}[], child: cp.ChildProcess, callback: (failures: number) => void, failures: number) {
+    const file = files.pop();
+    if (file) {
+        console.log(`Linting '${file.path}'.`);
+        child.send({ kind: "file", name: file.path });
+    }
+    else {
+        child.send({ kind: "close" });
+        callback(failures);
+    }
+}
+
+function spawnLintWorker(files: {path: string}[], callback: (failures: number) => void) {
+    const child = cp.fork("./scripts/parallel-lint");
+    let failures = 0;
+    child.on("message", function(data) {
+        switch (data.kind) {
+            case "result":
+                if (data.failures > 0) {
+                    failures += data.failures;
+                    console.log(data.output);
+                }
+                sendNextFile(files, child, callback, failures);
+                break;
+            case "error":
+                console.error(data.error);
+                failures++;
+                sendNextFile(files, child, callback, failures);
+                break;
+        }
+    });
+    sendNextFile(files, child, callback, failures);
+}
 
 gulp.task("lint", "Runs tslint on the compiler sources. Optional arguments are: --f[iles]=regex", ["build-rules"], () => {
     const fileMatcher = RegExp(cmdLineOptions["files"]);
-    const lintOptions = getLinterOptions();
-    let failed = 0;
-    return gulp.src(lintTargets)
-        .pipe(insert.transform((contents, file) => {
-            if (!fileMatcher.test(file.path)) return contents;
-            const result = lintFile(lintOptions, file.path);
-            if (result.failureCount > 0) {
-                console.log(result.output);
-                failed += result.failureCount;
+    if (fold.isTravis()) console.log(fold.start("lint"));
+
+    let files: {stat: fs.Stats, path: string}[] = [];
+    return gulp.src(lintTargets, { read: false })
+        .pipe(through2.obj((chunk, enc, cb) => {
+            files.push(chunk);
+            cb();
+        }, (cb) => {
+            files = files.filter(file =>  fileMatcher.test(file.path)).sort((filea, fileb) => filea.stat.size - fileb.stat.size);
+            const workerCount = (process.env.workerCount && +process.env.workerCount) || os.cpus().length;
+            for (let i = 0; i < workerCount; i++) {
+                spawnLintWorker(files, finished);
             }
-            return contents; // TODO (weswig): Automatically apply fixes? :3
-        }))
-        .on("end", () => {
-            if (failed > 0) {
-                console.error("Linter errors.");
-                process.exit(1);
+
+            let completed = 0;
+            let failures = 0;
+            function finished(fails) {
+                completed++;
+                failures += fails;
+                if (completed === workerCount) {
+                    if (fold.isTravis()) console.log(fold.end("lint"));
+                    if (failures > 0) {
+                        throw new Error(`Linter errors: ${failures}`);
+                    }
+                    else {
+                        cb();
+                    }
+                }
             }
-        });
+        }));
 });
 
 
diff --git a/Jakefile.js b/Jakefile.js
index 174be5e702f..e4aaf330dc7 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -4,7 +4,7 @@ var fs = require("fs");
 var os = require("os");
 var path = require("path");
 var child_process = require("child_process");
-var Linter = require("tslint");
+var fold = require("travis-fold");
 var runTestsInParallel = require("./scripts/mocha-parallel").runTestsInParallel;
 
 // Variables
@@ -32,6 +32,28 @@ if (process.env.path !== undefined) {
    process.env.PATH = nodeModulesPathPrefix + process.env.PATH;
 }
 
+function toNs(diff) {
+    return diff[0] * 1e9 + diff[1];
+}
+
+function mark() {
+    if (!fold.isTravis()) return;
+    var stamp = process.hrtime();
+    var id = Math.floor(Math.random() * 0xFFFFFFFF).toString(16);
+    console.log("travis_time:start:" + id + "\r");
+    return {
+        stamp: stamp,
+        id: id
+    };
+}
+
+function measure(marker) {
+    if (!fold.isTravis()) return;
+    var diff = process.hrtime(marker.stamp);
+    var total = [marker.stamp[0] + diff[0], marker.stamp[1] + diff[1]];
+    console.log("travis_time:end:" + marker.id + ":start=" + toNs(marker.stamp) + ",finish=" + toNs(total) + ",duration=" + toNs(diff) + "\r");
+}
+
 var compilerSources = [
     "core.ts",
     "performance.ts",
@@ -285,6 +307,7 @@ var builtLocalCompiler = path.join(builtLocalDirectory, compilerFilename);
     */
 function compileFile(outFile, sources, prereqs, prefixes, useBuiltCompiler, opts, callback) {
     file(outFile, prereqs, function() {
+        var startCompileTime = mark();
         opts = opts || {};
         var compilerPath = useBuiltCompiler ? builtLocalCompiler : LKGCompiler;
         var options = "--noImplicitAny --noImplicitThis --noEmitOnError --types " 
@@ -361,11 +384,13 @@ function compileFile(outFile, sources, prereqs, prefixes, useBuiltCompiler, opts
                 callback();
             }
 
+            measure(startCompileTime);
             complete();
         });
         ex.addListener("error", function() {
             fs.unlinkSync(outFile);
             fail("Compilation of " + outFile + " unsuccessful");
+            measure(startCompileTime);
         });
         ex.run();
     }, {async: true});
@@ -551,7 +576,7 @@ var tsserverLibraryDefinitionFile = path.join(builtLocalDirectory, "tsserverlibr
 compileFile(
     tsserverLibraryFile,
     languageServiceLibrarySources,
-    [builtLocalDirectory, copyright].concat(languageServiceLibrarySources),
+    [builtLocalDirectory, copyright, builtLocalCompiler].concat(languageServiceLibrarySources).concat(libraryTargets),
     /*prefixes*/ [copyright],
     /*useBuiltCompiler*/ true,
     { noOutFile: false, generateDeclarations: true });
@@ -560,9 +585,19 @@ compileFile(
 desc("Builds language service server library");
 task("lssl", [tsserverLibraryFile, tsserverLibraryDefinitionFile]);
 
+desc("Emit the start of the build fold");
+task("build-fold-start", [] , function() {
+    if (fold.isTravis()) console.log(fold.start("build"));
+});
+
+desc("Emit the end of the build fold");
+task("build-fold-end", [] , function() {
+    if (fold.isTravis()) console.log(fold.end("build"));
+});
+
 // Local target to build the compiler and services
 desc("Builds the full compiler and services");
-task("local", ["generate-diagnostics", "lib", tscFile, servicesFile, nodeDefinitionsFile, serverFile, builtGeneratedDiagnosticMessagesJSON]);
+task("local", ["build-fold-start", "generate-diagnostics", "lib", tscFile, servicesFile, nodeDefinitionsFile, serverFile, builtGeneratedDiagnosticMessagesJSON, "lssl", "build-fold-end"]);
 
 // Local target to build only tsc.js
 desc("Builds only the compiler");
@@ -617,7 +652,7 @@ task("generate-spec", [specMd]);
 
 // Makes a new LKG. This target does not build anything, but errors if not all the outputs are present in the built/local directory
 desc("Makes a new LKG out of the built js files");
-task("LKG", ["clean", "release", "local", "lssl"].concat(libraryTargets), function() {
+task("LKG", ["clean", "release", "local"].concat(libraryTargets), function() {
     var expectedFiles = [tscFile, servicesFile, serverFile, nodePackageFile, nodeDefinitionsFile, standaloneDefinitionsFile, tsserverLibraryFile, tsserverLibraryDefinitionFile].concat(libraryTargets);
     var missingFiles = expectedFiles.filter(function (f) {
         return !fs.existsSync(f);
@@ -758,6 +793,7 @@ function runConsoleTests(defaultReporter, runInParallel) {
     // timeout normally isn't necessary but Travis-CI has been timing out on compiler baselines occasionally
     // default timeout is 2sec which really should be enough, but maybe we just need a small amount longer
     if(!runInParallel) {
+        var startTime = mark();
         tests = tests ? ' -g "' + tests + '"' : '';
         var cmd = "mocha" + (debug ? " --debug-brk" : "") + " -R " + reporter + tests + colors + bail + ' -t ' + testTimeout + ' ' + run;
         console.log(cmd);
@@ -766,10 +802,12 @@ function runConsoleTests(defaultReporter, runInParallel) {
         process.env.NODE_ENV = "development";
         exec(cmd, function () {
             process.env.NODE_ENV = savedNodeEnv;
+            measure(startTime);
             runLinter();
             finish();
         }, function(e, status) {
             process.env.NODE_ENV = savedNodeEnv;
+            measure(startTime);
             finish(status);
         });
 
@@ -777,9 +815,10 @@ function runConsoleTests(defaultReporter, runInParallel) {
     else {
         var savedNodeEnv = process.env.NODE_ENV;
         process.env.NODE_ENV = "development";
+        var startTime = mark();
         runTestsInParallel(taskConfigsFolder, run, { testTimeout: testTimeout, noColors: colors === " --no-colors " }, function (err) {
             process.env.NODE_ENV = savedNodeEnv;
-
+            measure(startTime);
             // last worker clean everything and runs linter in case if there were no errors
             deleteTemporaryProjectOutput();
             jake.rmRf(taskConfigsFolder);
@@ -998,41 +1037,21 @@ var tslintRulesOutFiles = tslintRules.map(function(p) {
     return path.join(builtLocalDirectory, "tslint", p + ".js");
 });
 desc("Compiles tslint rules to js");
-task("build-rules", tslintRulesOutFiles);
+task("build-rules", ["build-rules-start"].concat(tslintRulesOutFiles).concat(["build-rules-end"]));
 tslintRulesFiles.forEach(function(ruleFile, i) {
     compileFile(tslintRulesOutFiles[i], [ruleFile], [ruleFile], [], /*useBuiltCompiler*/ false,
     { noOutFile: true, generateDeclarations: false, outDir: path.join(builtLocalDirectory, "tslint")});
 });
 
-function getLinterOptions() {
-    return {
-        configuration: require("./tslint.json"),
-        formatter: "prose",
-        formattersDirectory: undefined,
-        rulesDirectory: "built/local/tslint"
-    };
-}
+desc("Emit the start of the build-rules fold");
+task("build-rules-start", [] , function() {
+    if (fold.isTravis()) console.log(fold.start("build-rules"));
+});
 
-function lintFileContents(options, path, contents) {
-    var ll = new Linter(path, contents, options);
-    console.log("Linting '" + path + "'.");
-    return ll.lint();
-}
-
-function lintFile(options, path) {
-    var contents = fs.readFileSync(path, "utf8");
-    return lintFileContents(options, path, contents);
-}
-
-function lintFileAsync(options, path, cb) {
-    fs.readFile(path, "utf8", function(err, contents) {
-        if (err) {
-            return cb(err);
-        }
-        var result = lintFileContents(options, path, contents);
-        cb(undefined, result);
-    });
-}
+desc("Emit the end of the build-rules fold");
+task("build-rules-end", [] , function() {
+    if (fold.isTravis()) console.log(fold.end("build-rules"));
+});
 
 var lintTargets = compilerSources
     .concat(harnessSources)
@@ -1044,71 +1063,78 @@ var lintTargets = compilerSources
     .concat(["Gulpfile.ts"])
     .concat([nodeServerInFile, perftscPath, "tests/perfsys.ts", webhostPath]);
 
+function sendNextFile(files, child, callback, failures) {
+    var file = files.pop();
+    if (file) {
+        console.log("Linting '" + file + "'.");
+        child.send({kind: "file", name: file});
+    }
+    else {
+        child.send({kind: "close"});
+        callback(failures);
+    }
+}
+
+function spawnLintWorker(files, callback) {
+    var child = child_process.fork("./scripts/parallel-lint");
+    var failures = 0;
+    child.on("message", function(data) {
+        switch (data.kind) {
+            case "result":
+                if (data.failures > 0) {
+                    failures += data.failures;
+                    console.log(data.output);
+                }
+                sendNextFile(files, child, callback, failures);
+                break;
+            case "error":
+                console.error(data.error);
+                failures++;
+                sendNextFile(files, child, callback, failures);
+                break;
+        }
+    });
+    sendNextFile(files, child, callback, failures);
+}
 
 desc("Runs tslint on the compiler sources. Optional arguments are: f[iles]=regex");
 task("lint", ["build-rules"], function() {
-    var lintOptions = getLinterOptions();
+    if (fold.isTravis()) console.log(fold.start("lint"));
+    var startTime = mark();
     var failed = 0;
     var fileMatcher = RegExp(process.env.f || process.env.file || process.env.files || "");
     var done = {};
     for (var i in lintTargets) {
         var target = lintTargets[i];
         if (!done[target] && fileMatcher.test(target)) {
-            var result = lintFile(lintOptions, target);
-            if (result.failureCount > 0) {
-                console.log(result.output);
-                failed += result.failureCount;
-            }
-            done[target] = true;
+            done[target] = fs.statSync(target).size;
         }
     }
-    if (failed > 0) {
-        fail('Linter errors.', failed);
-    }
-});
 
-/**
- * This is required because file watches on Windows get fires _twice_
- * when a file changes on some node/windows version configuations
- * (node v4 and win 10, for example). By not running a lint for a file
- * which already has a pending lint, we avoid duplicating our work.
- * (And avoid printing duplicate results!)
- */
-var lintSemaphores = {};
+    var workerCount = (process.env.workerCount && +process.env.workerCount) || os.cpus().length;
 
-function lintWatchFile(filename) {
-    fs.watch(filename, {persistent: true}, function(event) {
-        if (event !== "change") {
-            return;
-        }
-
-        if (!lintSemaphores[filename]) {
-            lintSemaphores[filename] = true;
-            lintFileAsync(getLinterOptions(), filename, function(err, result) {
-                delete lintSemaphores[filename];
-                if (err) {
-                    console.log(err);
-                    return;
-                }
-                if (result.failureCount > 0) {
-                    console.log("***Lint failure***");
-                    for (var i = 0; i < result.failures.length; i++) {
-                        var failure = result.failures[i];
-                        var start = failure.startPosition.lineAndCharacter;
-                        var end = failure.endPosition.lineAndCharacter;
-                        console.log("warning " + filename + " (" + (start.line + 1) + "," + (start.character + 1) + "," + (end.line + 1) + "," + (end.character + 1) + "): " + failure.failure);
-                    }
-                    console.log("*** Total " + result.failureCount + " failures.");
-                }
-            });
-        }
+    var names = Object.keys(done).sort(function(namea, nameb) {
+        return done[namea] - done[nameb];
     });
-}
 
-desc("Watches files for changes to rerun a lint pass");
-task("lint-server", ["build-rules"], function() {
-    console.log("Watching ./src for changes to linted files");
-    for (var i = 0; i < lintTargets.length; i++) {
-        lintWatchFile(lintTargets[i]);
+    for (var i = 0; i < workerCount; i++) {
+        spawnLintWorker(names, finished);
     }
-});
+
+    var completed = 0;
+    var failures = 0;
+    function finished(fails) {
+        completed++;
+        failures += fails;
+        if (completed === workerCount) {
+            measure(startTime);
+            if (fold.isTravis()) console.log(fold.end("lint"));
+            if (failures > 0) {
+                fail('Linter errors.', failed);
+            }
+            else {
+                complete();
+            }
+        }
+    }
+}, {async: true});
diff --git a/README.md b/README.md
index fca2890bc77..d16bc363b26 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 
 [![Join the chat at https://gitter.im/Microsoft/TypeScript](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/Microsoft/TypeScript?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
-[TypeScript](http://www.typescriptlang.org/) is a language for application-scale JavaScript. TypeScript adds optional types, classes, and modules to JavaScript. TypeScript supports tools for large-scale JavaScript applications for any browser, for any host, on any OS. TypeScript compiles to readable, standards-based JavaScript. Try it out at the [playground](http://www.typescriptlang.org/Playground), and stay up to date via [our blog](http://blogs.msdn.com/typescript) and [Twitter account](https://twitter.com/typescriptlang).
+[TypeScript](http://www.typescriptlang.org/) is a language for application-scale JavaScript. TypeScript adds optional types, classes, and modules to JavaScript. TypeScript supports tools for large-scale JavaScript applications for any browser, for any host, on any OS. TypeScript compiles to readable, standards-based JavaScript. Try it out at the [playground](http://www.typescriptlang.org/Playground), and stay up to date via [our blog](https://blogs.msdn.microsoft.com/typescript) and [Twitter account](https://twitter.com/typescriptlang).
 
 ## Installing
 
diff --git a/package.json b/package.json
index 5606a423a0e..25d8f0c42ea 100644
--- a/package.json
+++ b/package.json
@@ -30,8 +30,8 @@
     },
     "devDependencies": {
         "@types/browserify": "latest",
-        "@types/convert-source-map": "latest",
         "@types/chai": "latest",
+        "@types/convert-source-map": "latest",
         "@types/del": "latest",
         "@types/glob": "latest",
         "@types/gulp": "latest",
@@ -72,13 +72,14 @@
         "run-sequence": "latest",
         "sorcery": "latest",
         "through2": "latest",
+        "travis-fold": "latest",
         "ts-node": "latest",
         "tslint": "next",
         "typescript": "next"
     },
     "scripts": {
         "pretest": "jake tests",
-        "test": "jake runtests",
+        "test": "jake runtests-parallel",
         "build": "npm run build:compiler && npm run build:tests",
         "build:compiler": "jake local",
         "build:tests": "jake tests",
diff --git a/scripts/parallel-lint.js b/scripts/parallel-lint.js
new file mode 100644
index 00000000000..a9aec06c2df
--- /dev/null
+++ b/scripts/parallel-lint.js
@@ -0,0 +1,45 @@
+var Linter = require("tslint");
+var fs = require("fs");
+
+function getLinterOptions() {
+    return {
+        configuration: require("../tslint.json"),
+        formatter: "prose",
+        formattersDirectory: undefined,
+        rulesDirectory: "built/local/tslint"
+    };
+}
+
+function lintFileContents(options, path, contents) {
+    var ll = new Linter(path, contents, options);
+    return ll.lint();
+}
+
+function lintFileAsync(options, path, cb) {
+    fs.readFile(path, "utf8", function (err, contents) {
+        if (err) {
+            return cb(err);
+        }
+        var result = lintFileContents(options, path, contents);
+        cb(undefined, result);
+    });
+}
+
+process.on("message", function (data) {
+    switch (data.kind) {
+        case "file":
+            var target = data.name;
+            var lintOptions = getLinterOptions();
+            lintFileAsync(lintOptions, target, function (err, result) {
+                if (err) {
+                    process.send({ kind: "error", error: err.toString() });
+                    return;
+                }
+                process.send({ kind: "result", failures: result.failureCount, output: result.output });
+            });
+            break;
+        case "close":
+            process.exit(0);
+            break;
+    }
+});
\ No newline at end of file
diff --git a/scripts/processDiagnosticMessages.ts b/scripts/processDiagnosticMessages.ts
index 26632ba6bab..431cf460180 100644
--- a/scripts/processDiagnosticMessages.ts
+++ b/scripts/processDiagnosticMessages.ts
@@ -69,7 +69,7 @@ function checkForUniqueCodes(messages: string[], diagnosticTable: InputDiagnosti
 }
 
 function buildUniqueNameMap(names: string[]): ts.Map<string> {
-    var nameMap: ts.Map<string> = {};
+    var nameMap = ts.createMap<string>();
 
     var uniqueNames = NameGenerator.ensureUniqueness(names, /* isCaseSensitive */ false, /* isFixed */ undefined);
 
diff --git a/scripts/tslint/preferConstRule.ts b/scripts/tslint/preferConstRule.ts
index aaa1b0e53d5..1d316692468 100644
--- a/scripts/tslint/preferConstRule.ts
+++ b/scripts/tslint/preferConstRule.ts
@@ -1,7 +1,6 @@
 import * as Lint from "tslint/lib/lint";
 import * as ts from "typescript";
 
-
 export class Rule extends Lint.Rules.AbstractRule {
     public static FAILURE_STRING_FACTORY = (identifier: string) => `Identifier '${identifier}' never appears on the LHS of an assignment - use const instead of let for its declaration.`;
 
@@ -64,7 +63,7 @@ interface DeclarationUsages {
 }
 
 class PreferConstWalker extends Lint.RuleWalker {
-    private inScopeLetDeclarations: ts.Map<DeclarationUsages>[] = [];
+    private inScopeLetDeclarations: ts.MapLike<DeclarationUsages>[] = [];
     private errors: Lint.RuleFailure[] = [];
     private markAssignment(identifier: ts.Identifier) {
         const name = identifier.text;
@@ -172,7 +171,7 @@ class PreferConstWalker extends Lint.RuleWalker {
     }
 
     private visitAnyForStatement(node: ts.ForOfStatement | ts.ForInStatement) {
-        const names: ts.Map<DeclarationUsages> = {};
+        const names: ts.MapLike<DeclarationUsages> = {};
         if (isLet(node.initializer)) {
             if (node.initializer.kind === ts.SyntaxKind.VariableDeclarationList) {
                 this.collectLetIdentifiers(node.initializer as ts.VariableDeclarationList, names);
@@ -194,7 +193,7 @@ class PreferConstWalker extends Lint.RuleWalker {
     }
 
     visitBlock(node: ts.Block) {
-        const names: ts.Map<DeclarationUsages> = {};
+        const names: ts.MapLike<DeclarationUsages> = {};
         for (const statement of node.statements) {
             if (statement.kind === ts.SyntaxKind.VariableStatement) {
                 this.collectLetIdentifiers((statement as ts.VariableStatement).declarationList, names);
@@ -205,7 +204,7 @@ class PreferConstWalker extends Lint.RuleWalker {
         this.popDeclarations();
     }
 
-    private collectLetIdentifiers(list: ts.VariableDeclarationList, ret: ts.Map<DeclarationUsages>) {
+    private collectLetIdentifiers(list: ts.VariableDeclarationList, ret: ts.MapLike<DeclarationUsages>) {
         for (const node of list.declarations) {
             if (isLet(node) && !isExported(node)) {
                 this.collectNameIdentifiers(node, node.name, ret);
@@ -213,7 +212,7 @@ class PreferConstWalker extends Lint.RuleWalker {
         }
     }
 
-    private collectNameIdentifiers(declaration: ts.VariableDeclaration, node: ts.Identifier | ts.BindingPattern, table: ts.Map<DeclarationUsages>) {
+    private collectNameIdentifiers(declaration: ts.VariableDeclaration, node: ts.Identifier | ts.BindingPattern, table: ts.MapLike<DeclarationUsages>) {
         if (node.kind === ts.SyntaxKind.Identifier) {
             table[(node as ts.Identifier).text] = { declaration, usages: 0 };
         }
@@ -222,7 +221,7 @@ class PreferConstWalker extends Lint.RuleWalker {
         }
     }
 
-    private collectBindingPatternIdentifiers(value: ts.VariableDeclaration, pattern: ts.BindingPattern, table: ts.Map<DeclarationUsages>) {
+    private collectBindingPatternIdentifiers(value: ts.VariableDeclaration, pattern: ts.BindingPattern, table: ts.MapLike<DeclarationUsages>) {
         for (const element of pattern.elements) {
             this.collectNameIdentifiers(value, element.name, table);
         }
diff --git a/scripts/types/ambient.d.ts b/scripts/types/ambient.d.ts
index e77e3fe8c5a..e83489801d7 100644
--- a/scripts/types/ambient.d.ts
+++ b/scripts/types/ambient.d.ts
@@ -22,3 +22,4 @@ declare module "into-stream" {
 }
 
 declare module "sorcery";
+declare module "travis-fold";
diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 849f503ef85..ee82a88d7a5 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -89,9 +89,10 @@ namespace ts {
     const binder = createBinder();
 
     export function bindSourceFile(file: SourceFile, options: CompilerOptions) {
-        const start = performance.mark();
+        performance.mark("beforeBind");
         binder(file, options);
-        performance.measure("Bind", start);
+        performance.mark("afterBind");
+        performance.measure("Bind", "beforeBind", "afterBind");
     }
 
     function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
@@ -135,7 +136,7 @@ namespace ts {
             options = opts;
             languageVersion = getEmitScriptTarget(options);
             inStrictMode = !!file.externalModuleIndicator;
-            classifiableNames = {};
+            classifiableNames = createMap<string>();
             symbolCount = 0;
 
             Symbol = objectAllocator.getSymbolConstructor();
@@ -183,11 +184,11 @@ namespace ts {
             symbol.declarations.push(node);
 
             if (symbolFlags & SymbolFlags.HasExports && !symbol.exports) {
-                symbol.exports = {};
+                symbol.exports = createMap<Symbol>();
             }
 
             if (symbolFlags & SymbolFlags.HasMembers && !symbol.members) {
-                symbol.members = {};
+                symbol.members = createMap<Symbol>();
             }
 
             if (symbolFlags & SymbolFlags.Value) {
@@ -325,9 +326,7 @@ namespace ts {
                 // Otherwise, we'll be merging into a compatible existing symbol (for example when
                 // you have multiple 'vars' with the same name in the same container).  In this case
                 // just add this node into the declarations list of the symbol.
-                symbol = hasProperty(symbolTable, name)
-                    ? symbolTable[name]
-                    : (symbolTable[name] = createSymbol(SymbolFlags.None, name));
+                symbol = symbolTable[name] || (symbolTable[name] = createSymbol(SymbolFlags.None, name));
 
                 if (name && (includes & SymbolFlags.Classifiable)) {
                     classifiableNames[name] = name;
@@ -445,7 +444,7 @@ namespace ts {
             if (containerFlags & ContainerFlags.IsContainer) {
                 container = blockScopeContainer = node;
                 if (containerFlags & ContainerFlags.HasLocals) {
-                    container.locals = {};
+                    container.locals = createMap<Symbol>();
                 }
                 addToContainerChain(container);
             }
@@ -1410,7 +1409,8 @@ namespace ts {
 
             const typeLiteralSymbol = createSymbol(SymbolFlags.TypeLiteral, "__type");
             addDeclarationToSymbol(typeLiteralSymbol, node, SymbolFlags.TypeLiteral);
-            typeLiteralSymbol.members = { [symbol.name]: symbol };
+            typeLiteralSymbol.members = createMap<Symbol>();
+            typeLiteralSymbol.members[symbol.name] = symbol;
         }
 
         function bindObjectLiteralExpression(node: ObjectLiteralExpression) {
@@ -1420,7 +1420,7 @@ namespace ts {
             }
 
             if (inStrictMode) {
-                const seen: Map<ElementKind> = {};
+                const seen = createMap<ElementKind>();
 
                 for (const prop of node.properties) {
                     if (prop.name.kind !== SyntaxKind.Identifier) {
@@ -1476,7 +1476,7 @@ namespace ts {
                 // fall through.
                 default:
                     if (!blockScopeContainer.locals) {
-                        blockScopeContainer.locals = {};
+                        blockScopeContainer.locals = createMap<Symbol>();
                         addToContainerChain(blockScopeContainer);
                     }
                     declareSymbol(blockScopeContainer.locals, undefined, node, symbolFlags, symbolExcludes);
@@ -1898,18 +1898,17 @@ namespace ts {
         }
 
         function bindExportAssignment(node: ExportAssignment | BinaryExpression) {
-            const boundExpression = node.kind === SyntaxKind.ExportAssignment ? (<ExportAssignment>node).expression : (<BinaryExpression>node).right;
             if (!container.symbol || !container.symbol.exports) {
                 // Export assignment in some sort of block construct
                 bindAnonymousDeclaration(node, SymbolFlags.Alias, getDeclarationName(node));
             }
-            else if (boundExpression.kind === SyntaxKind.Identifier && node.kind === SyntaxKind.ExportAssignment) {
-                // An export default clause with an identifier exports all meanings of that identifier
-                declareSymbol(container.symbol.exports, container.symbol, node, SymbolFlags.Alias, SymbolFlags.PropertyExcludes | SymbolFlags.AliasExcludes);
-            }
             else {
-                // An export default clause with an expression exports a value
-                declareSymbol(container.symbol.exports, container.symbol, node, SymbolFlags.Property, SymbolFlags.PropertyExcludes | SymbolFlags.AliasExcludes);
+                const flags = node.kind === SyntaxKind.ExportAssignment && exportAssignmentIsAlias(<ExportAssignment>node)
+                    // An export default clause with an EntityNameExpression exports all meanings of that identifier
+                    ? SymbolFlags.Alias
+                    // An export default clause with any other expression exports a value
+                    : SymbolFlags.Property;
+                declareSymbol(container.symbol.exports, container.symbol, node, flags, SymbolFlags.PropertyExcludes | SymbolFlags.AliasExcludes);
             }
         }
 
@@ -1936,7 +1935,7 @@ namespace ts {
                 }
             }
 
-            file.symbol.globalExports = file.symbol.globalExports || {};
+            file.symbol.globalExports = file.symbol.globalExports || createMap<Symbol>();
             declareSymbol(file.symbol.globalExports, file.symbol, node, SymbolFlags.Alias, SymbolFlags.AliasExcludes);
         }
 
@@ -1984,12 +1983,23 @@ namespace ts {
                 assignee = container;
             }
             else if (container.kind === SyntaxKind.Constructor) {
-                assignee = container.parent;
+                if (isInJavaScriptFile(node)) {
+                    // this.foo assignment in a JavaScript class
+                    // Bind this property to the containing class
+                    const saveContainer = container;
+                    container = container.parent;
+                    bindPropertyOrMethodOrAccessor(node, SymbolFlags.Property, SymbolFlags.None);
+                    container = saveContainer;
+                    return;
+                }
+                else {
+                    assignee = container.parent;
+                }
             }
             else {
                 return;
             }
-            assignee.symbol.members = assignee.symbol.members || {};
+            assignee.symbol.members = assignee.symbol.members || createMap<Symbol>();
             // It's acceptable for multiple 'this' assignments of the same identifier to occur
             // AND it can be overwritten by subsequent method declarations
             const symbol = declareSymbol(assignee.symbol.members, assignee.symbol, node, SymbolFlags.Property, SymbolFlags.PropertyExcludes & ~SymbolFlags.Property);
@@ -2017,7 +2027,7 @@ namespace ts {
 
             // Set up the members collection if it doesn't exist already
             if (!funcSymbol.members) {
-                funcSymbol.members = {};
+                funcSymbol.members = createMap<Symbol>();
             }
 
             // Declare the method/property
@@ -2066,7 +2076,7 @@ namespace ts {
             // module might have an exported variable called 'prototype'.  We can't allow that as
             // that would clash with the built-in 'prototype' for the class.
             const prototypeSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Prototype, "prototype");
-            if (hasProperty(symbol.exports, prototypeSymbol.name)) {
+            if (symbol.exports[prototypeSymbol.name]) {
                 if (node.name) {
                     node.name.parent = node;
                 }
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c1626ff0b75..3d7fb7eb7e1 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -44,7 +44,7 @@ namespace ts {
         let symbolCount = 0;
 
         const emptyArray: any[] = [];
-        const emptySymbols: SymbolTable = {};
+        const emptySymbols = createMap<Symbol>();
 
         const compilerOptions = host.getCompilerOptions();
         const languageVersion = compilerOptions.target || ScriptTarget.ES3;
@@ -106,11 +106,11 @@ namespace ts {
             isOptionalParameter
         };
 
-        const tupleTypes: Map<TupleType> = {};
-        const unionTypes: Map<UnionType> = {};
-        const intersectionTypes: Map<IntersectionType> = {};
-        const stringLiteralTypes: Map<LiteralType> = {};
-        const numericLiteralTypes: Map<LiteralType> = {};
+        const tupleTypes = createMap<TupleType>();
+        const unionTypes = createMap<UnionType>();
+        const intersectionTypes = createMap<IntersectionType>();
+        const stringLiteralTypes = createMap<LiteralType>();
+        const numericLiteralTypes = createMap<LiteralType>();
 
         const unknownSymbol = createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "unknown");
         const resolvingSymbol = createSymbol(SymbolFlags.Transient, "__resolving__");
@@ -132,7 +132,7 @@ namespace ts {
 
         const emptyObjectType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
         const emptyGenericType = <GenericType><ObjectType>createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
-        emptyGenericType.instantiations = {};
+        emptyGenericType.instantiations = createMap<TypeReference>();
 
         const anyFunctionType = createAnonymousType(undefined, emptySymbols, emptyArray, emptyArray, undefined, undefined);
         // The anyFunctionType contains the anyFunctionType by definition. The flag is further propagated
@@ -146,7 +146,7 @@ namespace ts {
 
         const enumNumberIndexInfo = createIndexInfo(stringType, /*isReadonly*/ true);
 
-        const globals: SymbolTable = {};
+        const globals = createMap<Symbol>();
         /**
          * List of every ambient module with a "*" wildcard.
          * Unlike other ambient modules, these can't be stored in `globals` because symbol tables only deal with exact matches.
@@ -215,7 +215,7 @@ namespace ts {
         const flowLoopKeys: string[] = [];
         const flowLoopTypes: Type[][] = [];
         const visitedFlowNodes: FlowNode[] = [];
-        const visitedFlowTypes: Type[] = [];
+        const visitedFlowTypes: FlowType[] = [];
         const potentialThisCollisions: Node[] = [];
         const awaitedTypeStack: number[] = [];
 
@@ -245,7 +245,8 @@ namespace ts {
             NEUndefinedOrNull = 1 << 19,  // x != undefined / x != null
             Truthy = 1 << 20,             // x
             Falsy = 1 << 21,              // !x
-            All = (1 << 22) - 1,
+            Discriminatable = 1 << 22,    // May have discriminant property
+            All = (1 << 23) - 1,
             // The following members encode facts about particular kinds of types for use in the getTypeFacts function.
             // The presence of a particular fact means that the given test is true for some (and possibly all) values
             // of that kind of type.
@@ -275,15 +276,15 @@ namespace ts {
             TrueFacts = BaseBooleanFacts | Truthy,
             SymbolStrictFacts = TypeofEQSymbol | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
             SymbolFacts = SymbolStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
-            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
+            ObjectStrictFacts = TypeofEQObject | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
             ObjectFacts = ObjectStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
-            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy,
+            FunctionStrictFacts = TypeofEQFunction | TypeofEQHostObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | NEUndefined | NENull | NEUndefinedOrNull | Truthy | Discriminatable,
             FunctionFacts = FunctionStrictFacts | EQUndefined | EQNull | EQUndefinedOrNull | Falsy,
             UndefinedFacts = TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEObject | TypeofNEFunction | TypeofNEHostObject | EQUndefined | EQUndefinedOrNull | NENull | Falsy,
             NullFacts = TypeofEQObject | TypeofNEString | TypeofNENumber | TypeofNEBoolean | TypeofNESymbol | TypeofNEFunction | TypeofNEHostObject | EQNull | EQUndefinedOrNull | NEUndefined | Falsy,
         }
 
-        const typeofEQFacts: Map<TypeFacts> = {
+        const typeofEQFacts = createMap({
             "string": TypeFacts.TypeofEQString,
             "number": TypeFacts.TypeofEQNumber,
             "boolean": TypeFacts.TypeofEQBoolean,
@@ -291,9 +292,9 @@ namespace ts {
             "undefined": TypeFacts.EQUndefined,
             "object": TypeFacts.TypeofEQObject,
             "function": TypeFacts.TypeofEQFunction
-        };
+        });
 
-        const typeofNEFacts: Map<TypeFacts> = {
+        const typeofNEFacts = createMap({
             "string": TypeFacts.TypeofNEString,
             "number": TypeFacts.TypeofNENumber,
             "boolean": TypeFacts.TypeofNEBoolean,
@@ -301,19 +302,19 @@ namespace ts {
             "undefined": TypeFacts.NEUndefined,
             "object": TypeFacts.TypeofNEObject,
             "function": TypeFacts.TypeofNEFunction
-        };
+        });
 
-        const typeofTypesByName: Map<Type> = {
+        const typeofTypesByName = createMap<Type>({
             "string": stringType,
             "number": numberType,
             "boolean": booleanType,
             "symbol": esSymbolType,
             "undefined": undefinedType
-        };
+        });
 
         let jsxElementType: ObjectType;
         /** Things we lazy load from the JSX namespace */
-        const jsxTypes: Map<ObjectType> = {};
+        const jsxTypes = createMap<ObjectType>();
         const JsxNames = {
             JSX: "JSX",
             IntrinsicElements: "IntrinsicElements",
@@ -324,10 +325,10 @@ namespace ts {
             IntrinsicClassAttributes: "IntrinsicClassAttributes"
         };
 
-        const subtypeRelation: Map<RelationComparisonResult> = {};
-        const assignableRelation: Map<RelationComparisonResult> = {};
-        const comparableRelation: Map<RelationComparisonResult> = {};
-        const identityRelation: Map<RelationComparisonResult> = {};
+        const subtypeRelation = createMap<RelationComparisonResult>();
+        const assignableRelation = createMap<RelationComparisonResult>();
+        const comparableRelation = createMap<RelationComparisonResult>();
+        const identityRelation = createMap<RelationComparisonResult>();
 
         // This is for caching the result of getSymbolDisplayBuilder. Do not access directly.
         let _displayBuilder: SymbolDisplayBuilder;
@@ -341,9 +342,8 @@ namespace ts {
             ResolvedReturnType
         }
 
-        const builtinGlobals: SymbolTable = {
-            [undefinedSymbol.name]: undefinedSymbol
-        };
+        const builtinGlobals = createMap<Symbol>();
+        builtinGlobals[undefinedSymbol.name] = undefinedSymbol;
 
         initializeTypeChecker();
 
@@ -403,8 +403,8 @@ namespace ts {
             result.parent = symbol.parent;
             if (symbol.valueDeclaration) result.valueDeclaration = symbol.valueDeclaration;
             if (symbol.constEnumOnlyModule) result.constEnumOnlyModule = true;
-            if (symbol.members) result.members = cloneSymbolTable(symbol.members);
-            if (symbol.exports) result.exports = cloneSymbolTable(symbol.exports);
+            if (symbol.members) result.members = cloneMap(symbol.members);
+            if (symbol.exports) result.exports = cloneMap(symbol.exports);
             recordMergedSymbol(result, symbol);
             return result;
         }
@@ -426,11 +426,11 @@ namespace ts {
                     target.declarations.push(node);
                 });
                 if (source.members) {
-                    if (!target.members) target.members = {};
+                    if (!target.members) target.members = createMap<Symbol>();
                     mergeSymbolTable(target.members, source.members);
                 }
                 if (source.exports) {
-                    if (!target.exports) target.exports = {};
+                    if (!target.exports) target.exports = createMap<Symbol>();
                     mergeSymbolTable(target.exports, source.exports);
                 }
                 recordMergedSymbol(target, source);
@@ -447,29 +447,17 @@ namespace ts {
             }
         }
 
-        function cloneSymbolTable(symbolTable: SymbolTable): SymbolTable {
-            const result: SymbolTable = {};
-            for (const id in symbolTable) {
-                if (hasProperty(symbolTable, id)) {
-                    result[id] = symbolTable[id];
-                }
-            }
-            return result;
-        }
-
         function mergeSymbolTable(target: SymbolTable, source: SymbolTable) {
             for (const id in source) {
-                if (hasProperty(source, id)) {
-                    if (!hasProperty(target, id)) {
-                        target[id] = source[id];
-                    }
-                    else {
-                        let symbol = target[id];
-                        if (!(symbol.flags & SymbolFlags.Merged)) {
-                            target[id] = symbol = cloneSymbol(symbol);
-                        }
-                        mergeSymbol(symbol, source[id]);
+                let targetSymbol = target[id];
+                if (!targetSymbol) {
+                    target[id] = source[id];
+                }
+                else {
+                    if (!(targetSymbol.flags & SymbolFlags.Merged)) {
+                        target[id] = targetSymbol = cloneSymbol(targetSymbol);
                     }
+                    mergeSymbol(targetSymbol, source[id]);
                 }
             }
         }
@@ -513,14 +501,12 @@ namespace ts {
 
         function addToSymbolTable(target: SymbolTable, source: SymbolTable, message: DiagnosticMessage) {
             for (const id in source) {
-                if (hasProperty(source, id)) {
-                    if (hasProperty(target, id)) {
-                        // Error on redeclarations
-                        forEach(target[id].declarations, addDeclarationDiagnostic(id, message));
-                    }
-                    else {
-                        target[id] = source[id];
-                    }
+                if (target[id]) {
+                    // Error on redeclarations
+                    forEach(target[id].declarations, addDeclarationDiagnostic(id, message));
+                }
+                else {
+                    target[id] = source[id];
                 }
             }
 
@@ -537,7 +523,7 @@ namespace ts {
 
         function getNodeLinks(node: Node): NodeLinks {
             const nodeId = getNodeId(node);
-            return nodeLinks[nodeId] || (nodeLinks[nodeId] = {});
+            return nodeLinks[nodeId] || (nodeLinks[nodeId] = { flags: 0 });
         }
 
         function isGlobalSourceFile(node: Node) {
@@ -545,18 +531,20 @@ namespace ts {
         }
 
         function getSymbol(symbols: SymbolTable, name: string, meaning: SymbolFlags): Symbol {
-            if (meaning && hasProperty(symbols, name)) {
+            if (meaning) {
                 const symbol = symbols[name];
-                Debug.assert((symbol.flags & SymbolFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
-                if (symbol.flags & meaning) {
-                    return symbol;
-                }
-                if (symbol.flags & SymbolFlags.Alias) {
-                    const target = resolveAlias(symbol);
-                    // Unknown symbol means an error occurred in alias resolution, treat it as positive answer to avoid cascading errors
-                    if (target === unknownSymbol || target.flags & meaning) {
+                if (symbol) {
+                    Debug.assert((symbol.flags & SymbolFlags.Instantiated) === 0, "Should never get an instantiated symbol here.");
+                    if (symbol.flags & meaning) {
                         return symbol;
                     }
+                    if (symbol.flags & SymbolFlags.Alias) {
+                        const target = resolveAlias(symbol);
+                        // Unknown symbol means an error occurred in alias resolution, treat it as positive answer to avoid cascading errors
+                        if (target === unknownSymbol || target.flags & meaning) {
+                            return symbol;
+                        }
+                    }
                 }
             }
             // return undefined if we can't find a symbol.
@@ -664,7 +652,7 @@ namespace ts {
         // Resolve a given name for a given meaning at a given location. An error is reported if the name was not found and
         // the nameNotFoundMessage argument is not undefined. Returns the resolved symbol, or undefined if no symbol with
         // the given name can be found.
-        function resolveName(location: Node, name: string, meaning: SymbolFlags, nameNotFoundMessage: DiagnosticMessage, nameArg: string | Identifier): Symbol {
+        function resolveName(location: Node | undefined, name: string, meaning: SymbolFlags, nameNotFoundMessage: DiagnosticMessage, nameArg: string | Identifier): Symbol {
             let result: Symbol;
             let lastLocation: Node;
             let propertyWithInvalidInitializer: Node;
@@ -744,7 +732,7 @@ namespace ts {
                             //     2. We check === SymbolFlags.Alias in order to check that the symbol is *purely*
                             //        an alias. If we used &, we'd be throwing out symbols that have non alias aspects,
                             //        which is not the desired behavior.
-                            if (hasProperty(moduleExports, name) &&
+                            if (moduleExports[name] &&
                                 moduleExports[name].flags === SymbolFlags.Alias &&
                                 getDeclarationOfKind(moduleExports[name], SyntaxKind.ExportSpecifier)) {
                                 break;
@@ -881,7 +869,8 @@ namespace ts {
 
             if (!result) {
                 if (nameNotFoundMessage) {
-                    if (!checkAndReportErrorForMissingPrefix(errorLocation, name, nameArg) &&
+                    if (!errorLocation ||
+                        !checkAndReportErrorForMissingPrefix(errorLocation, name, nameArg) &&
                         !checkAndReportErrorForExtendingInterface(errorLocation)) {
                         error(errorLocation, nameNotFoundMessage, typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg));
                     }
@@ -930,7 +919,7 @@ namespace ts {
         }
 
         function checkAndReportErrorForMissingPrefix(errorLocation: Node, name: string, nameArg: string | Identifier): boolean {
-            if (!errorLocation || (errorLocation.kind === SyntaxKind.Identifier && (isTypeReferenceIdentifier(<Identifier>errorLocation)) || isInTypeQuery(errorLocation))) {
+            if ((errorLocation.kind === SyntaxKind.Identifier && (isTypeReferenceIdentifier(<Identifier>errorLocation)) || isInTypeQuery(errorLocation))) {
                 return false;
             }
 
@@ -968,28 +957,30 @@ namespace ts {
 
 
         function checkAndReportErrorForExtendingInterface(errorLocation: Node): boolean {
-            let parentClassExpression = errorLocation;
-            while (parentClassExpression) {
-                const kind = parentClassExpression.kind;
-                if (kind === SyntaxKind.Identifier || kind === SyntaxKind.PropertyAccessExpression) {
-                    parentClassExpression = parentClassExpression.parent;
-                    continue;
-                }
-                if (kind === SyntaxKind.ExpressionWithTypeArguments) {
-                    break;
-                }
-                return false;
-            }
-            if (!parentClassExpression) {
-                return false;
-            }
-            const expression = (<ExpressionWithTypeArguments>parentClassExpression).expression;
-            if (resolveEntityName(expression, SymbolFlags.Interface, /*ignoreErrors*/ true)) {
+            const expression = getEntityNameForExtendingInterface(errorLocation);
+            const isError = !!(expression && resolveEntityName(expression, SymbolFlags.Interface, /*ignoreErrors*/ true));
+            if (isError) {
                 error(errorLocation, Diagnostics.Cannot_extend_an_interface_0_Did_you_mean_implements, getTextOfNode(expression));
-                return true;
             }
-            return false;
+            return isError;
         }
+        /**
+         * Climbs up parents to an ExpressionWithTypeArguments, and returns its expression,
+         * but returns undefined if that expression is not an EntityNameExpression.
+         */
+        function getEntityNameForExtendingInterface(node: Node): EntityNameExpression | undefined {
+            switch (node.kind) {
+                case SyntaxKind.Identifier:
+                case SyntaxKind.PropertyAccessExpression:
+                    return node.parent ? getEntityNameForExtendingInterface(node.parent) : undefined;
+                case SyntaxKind.ExpressionWithTypeArguments:
+                    Debug.assert(isEntityNameExpression((<ExpressionWithTypeArguments>node).expression));
+                    return <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression;
+                default:
+                    return undefined;
+            }
+        }
+
 
         function checkResolvedBlockScopedVariable(result: Symbol, errorLocation: Node): void {
             Debug.assert((result.flags & SymbolFlags.BlockScopedVariable) !== 0);
@@ -1033,7 +1024,7 @@ namespace ts {
         }
 
         function getDeclarationOfAliasSymbol(symbol: Symbol): Declaration {
-            return forEach(symbol.declarations, d => isAliasSymbolDeclaration(d) ? d : undefined);
+            return find(symbol.declarations, d => isAliasSymbolDeclaration(d) ? d : undefined);
         }
 
         function getTargetOfImportEqualsDeclaration(node: ImportEqualsDeclaration): Symbol {
@@ -1101,9 +1092,9 @@ namespace ts {
 
         function getExportOfModule(symbol: Symbol, name: string): Symbol {
             if (symbol.flags & SymbolFlags.Module) {
-                const exports = getExportsOfSymbol(symbol);
-                if (hasProperty(exports, name)) {
-                    return resolveSymbol(exports[name]);
+                const exportedSymbol = getExportsOfSymbol(symbol)[name];
+                if (exportedSymbol) {
+                    return resolveSymbol(exportedSymbol);
                 }
             }
         }
@@ -1168,7 +1159,7 @@ namespace ts {
         }
 
         function getTargetOfExportAssignment(node: ExportAssignment): Symbol {
-            return resolveEntityName(<Identifier>node.expression, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace);
+            return resolveEntityName(<EntityNameExpression>node.expression, SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace);
         }
 
         function getTargetOfAliasDeclaration(node: Declaration): Symbol {
@@ -1278,7 +1269,7 @@ namespace ts {
         }
 
         // Resolves a qualified name and any involved aliases
-        function resolveEntityName(name: EntityName | Expression, meaning: SymbolFlags, ignoreErrors?: boolean, dontResolveAlias?: boolean): Symbol {
+        function resolveEntityName(name: EntityNameOrEntityNameExpression, meaning: SymbolFlags, ignoreErrors?: boolean, dontResolveAlias?: boolean): Symbol | undefined {
             if (nodeIsMissing(name)) {
                 return undefined;
             }
@@ -1293,7 +1284,7 @@ namespace ts {
                 }
             }
             else if (name.kind === SyntaxKind.QualifiedName || name.kind === SyntaxKind.PropertyAccessExpression) {
-                const left = name.kind === SyntaxKind.QualifiedName ? (<QualifiedName>name).left : (<PropertyAccessExpression>name).expression;
+                const left = name.kind === SyntaxKind.QualifiedName ? (<QualifiedName>name).left : (<PropertyAccessEntityNameExpression>name).expression;
                 const right = name.kind === SyntaxKind.QualifiedName ? (<QualifiedName>name).right : (<PropertyAccessExpression>name).name;
 
                 const namespace = resolveEntityName(left, SymbolFlags.Namespace, ignoreErrors);
@@ -1421,7 +1412,7 @@ namespace ts {
          */
         function extendExportSymbols(target: SymbolTable, source: SymbolTable, lookupTable?: Map<ExportCollisionTracker>, exportNode?: ExportDeclaration) {
             for (const id in source) {
-                if (id !== "default" && !hasProperty(target, id)) {
+                if (id !== "default" && !target[id]) {
                     target[id] = source[id];
                     if (lookupTable && exportNode) {
                         lookupTable[id] = {
@@ -1429,7 +1420,7 @@ namespace ts {
                         } as ExportCollisionTracker;
                     }
                 }
-                else if (lookupTable && exportNode && id !== "default" && hasProperty(target, id) && resolveSymbol(target[id]) !== resolveSymbol(source[id])) {
+                else if (lookupTable && exportNode && id !== "default" && target[id] && resolveSymbol(target[id]) !== resolveSymbol(source[id])) {
                     if (!lookupTable[id].exportsWithDuplicate) {
                         lookupTable[id].exportsWithDuplicate = [exportNode];
                     }
@@ -1451,12 +1442,12 @@ namespace ts {
                     return;
                 }
                 visitedSymbols.push(symbol);
-                const symbols = cloneSymbolTable(symbol.exports);
+                const symbols = cloneMap(symbol.exports);
                 // All export * declarations are collected in an __export symbol by the binder
                 const exportStars = symbol.exports["__export"];
                 if (exportStars) {
-                    const nestedSymbols: SymbolTable = {};
-                    const lookupTable: Map<ExportCollisionTracker> = {};
+                    const nestedSymbols = createMap<Symbol>();
+                    const lookupTable = createMap<ExportCollisionTracker>();
                     for (const node of exportStars.declarations) {
                         const resolvedModule = resolveExternalModuleName(node, (node as ExportDeclaration).moduleSpecifier);
                         const exportedSymbols = visit(resolvedModule);
@@ -1470,7 +1461,7 @@ namespace ts {
                     for (const id in lookupTable) {
                         const { exportsWithDuplicate } = lookupTable[id];
                         // It's not an error if the file with multiple `export *`s with duplicate names exports a member with that name itself
-                        if (id === "export=" || !(exportsWithDuplicate && exportsWithDuplicate.length) || hasProperty(symbols, id)) {
+                        if (id === "export=" || !(exportsWithDuplicate && exportsWithDuplicate.length) || symbols[id]) {
                             continue;
                         }
                         for (const node of exportsWithDuplicate) {
@@ -1538,8 +1529,8 @@ namespace ts {
 
         function createType(flags: TypeFlags): Type {
             const result = new Type(checker, flags);
-            result.id = typeCount;
             typeCount++;
+            result.id = typeCount;
             return result;
         }
 
@@ -1576,13 +1567,11 @@ namespace ts {
         function getNamedMembers(members: SymbolTable): Symbol[] {
             let result: Symbol[];
             for (const id in members) {
-                if (hasProperty(members, id)) {
-                    if (!isReservedMemberName(id)) {
-                        if (!result) result = [];
-                        const symbol = members[id];
-                        if (symbolIsValue(symbol)) {
-                            result.push(symbol);
-                        }
+                if (!isReservedMemberName(id)) {
+                    if (!result) result = [];
+                    const symbol = members[id];
+                    if (symbolIsValue(symbol)) {
+                        result.push(symbol);
                     }
                 }
             }
@@ -1658,12 +1647,12 @@ namespace ts {
                 }
 
                 // If symbol is directly available by its name in the symbol table
-                if (isAccessible(lookUp(symbols, symbol.name))) {
+                if (isAccessible(symbols[symbol.name])) {
                     return [symbol];
                 }
 
                 // Check if symbol is any of the alias
-                return forEachValue(symbols, symbolFromSymbolTable => {
+                return forEachProperty(symbols, symbolFromSymbolTable => {
                     if (symbolFromSymbolTable.flags & SymbolFlags.Alias
                         && symbolFromSymbolTable.name !== "export="
                         && !getDeclarationOfKind(symbolFromSymbolTable, SyntaxKind.ExportSpecifier)) {
@@ -1698,12 +1687,12 @@ namespace ts {
             let qualify = false;
             forEachSymbolTableInScope(enclosingDeclaration, symbolTable => {
                 // If symbol of this name is not available in the symbol table we are ok
-                if (!hasProperty(symbolTable, symbol.name)) {
+                let symbolFromSymbolTable = symbolTable[symbol.name];
+                if (!symbolFromSymbolTable) {
                     // Continue to the next symbol table
                     return false;
                 }
                 // If the symbol with this name is present it should refer to the symbol
-                let symbolFromSymbolTable = symbolTable[symbol.name];
                 if (symbolFromSymbolTable === symbol) {
                     // No need to qualify
                     return true;
@@ -1849,7 +1838,7 @@ namespace ts {
             }
         }
 
-        function isEntityNameVisible(entityName: EntityName | Expression, enclosingDeclaration: Node): SymbolVisibilityResult {
+        function isEntityNameVisible(entityName: EntityNameOrEntityNameExpression, enclosingDeclaration: Node): SymbolVisibilityResult {
             // get symbol of the first identifier of the entityName
             let meaning: SymbolFlags;
             if (entityName.parent.kind === SyntaxKind.TypeQuery || isExpressionWithTypeArgumentsInClassExtendsClause(entityName.parent)) {
@@ -3121,7 +3110,7 @@ namespace ts {
 
         // Return the type implied by an object binding pattern
         function getTypeFromObjectBindingPattern(pattern: BindingPattern, includePatternInType: boolean): Type {
-            const members: SymbolTable = {};
+            const members = createMap<Symbol>();
             let hasComputedProperties = false;
             forEach(pattern.elements, e => {
                 const name = e.propertyName || <Identifier>e.name;
@@ -3251,6 +3240,13 @@ namespace ts {
                 // * className.prototype.method = expr
                 if (declaration.kind === SyntaxKind.BinaryExpression ||
                     declaration.kind === SyntaxKind.PropertyAccessExpression && declaration.parent.kind === SyntaxKind.BinaryExpression) {
+                    // Use JS Doc type if present on parent expression statement
+                    if (declaration.flags & NodeFlags.JavaScriptFile) {
+                        const typeTag = getJSDocTypeTag(declaration.parent);
+                        if (typeTag && typeTag.typeExpression) {
+                            return links.type = getTypeFromTypeNode(typeTag.typeExpression.type);
+                        }
+                    }
                     const declaredTypes = map(symbol.declarations,
                         decl => decl.kind === SyntaxKind.BinaryExpression ?
                             checkExpressionCached((<BinaryExpression>decl).right) :
@@ -3679,7 +3675,7 @@ namespace ts {
                     const baseTypeNodes = getInterfaceBaseTypeNodes(<InterfaceDeclaration>declaration);
                     if (baseTypeNodes) {
                         for (const node of baseTypeNodes) {
-                            if (isSupportedExpressionWithTypeArguments(node)) {
+                            if (isEntityNameExpression(node.expression)) {
                                 const baseSymbol = resolveEntityName(node.expression, SymbolFlags.Type, /*ignoreErrors*/ true);
                                 if (!baseSymbol || !(baseSymbol.flags & SymbolFlags.Interface) || getDeclaredTypeOfClassOrInterface(baseSymbol).thisType) {
                                     return false;
@@ -3709,7 +3705,7 @@ namespace ts {
                     type.typeParameters = concatenate(outerTypeParameters, localTypeParameters);
                     type.outerTypeParameters = outerTypeParameters;
                     type.localTypeParameters = localTypeParameters;
-                    (<GenericType>type).instantiations = {};
+                    (<GenericType>type).instantiations = createMap<TypeReference>();
                     (<GenericType>type).instantiations[getTypeListId(type.typeParameters)] = <GenericType>type;
                     (<GenericType>type).target = <GenericType>type;
                     (<GenericType>type).typeArguments = type.typeParameters;
@@ -3751,7 +3747,7 @@ namespace ts {
                     if (typeParameters) {
                         // Initialize the instantiation cache for generic type aliases. The declared type corresponds to
                         // an instantiation of the type alias with the type parameters supplied as type arguments.
-                        links.instantiations = {};
+                        links.instantiations = createMap<Type>();
                         links.instantiations[getTypeListId(links.typeParameters)] = type;
                     }
                 }
@@ -3772,7 +3768,7 @@ namespace ts {
             return expr.kind === SyntaxKind.NumericLiteral ||
                 expr.kind === SyntaxKind.PrefixUnaryExpression && (<PrefixUnaryExpression>expr).operator === SyntaxKind.MinusToken &&
                 (<PrefixUnaryExpression>expr).operand.kind === SyntaxKind.NumericLiteral ||
-                expr.kind === SyntaxKind.Identifier && hasProperty(symbol.exports, (<Identifier>expr).text);
+                expr.kind === SyntaxKind.Identifier && !!symbol.exports[(<Identifier>expr).text];
         }
 
         function enumHasLiteralMembers(symbol: Symbol) {
@@ -3795,7 +3791,7 @@ namespace ts {
                 enumType.symbol = symbol;
                 if (enumHasLiteralMembers(symbol)) {
                     const memberTypeList: Type[] = [];
-                    const memberTypes: Map<EnumLiteralType> = {};
+                    const memberTypes = createMap<EnumLiteralType>();
                     for (const declaration of enumType.symbol.declarations) {
                         if (declaration.kind === SyntaxKind.EnumDeclaration) {
                             computeEnumMemberValues(<EnumDeclaration>declaration);
@@ -3958,7 +3954,7 @@ namespace ts {
         }
 
         function createSymbolTable(symbols: Symbol[]): SymbolTable {
-            const result: SymbolTable = {};
+            const result = createMap<Symbol>();
             for (const symbol of symbols) {
                 result[symbol.name] = symbol;
             }
@@ -3968,7 +3964,7 @@ namespace ts {
         // The mappingThisOnly flag indicates that the only type parameter being mapped is "this". When the flag is true,
         // we check symbols to see if we can quickly conclude they are free of "this" references, thus needing no instantiation.
         function createInstantiatedSymbolTable(symbols: Symbol[], mapper: TypeMapper, mappingThisOnly: boolean): SymbolTable {
-            const result: SymbolTable = {};
+            const result = createMap<Symbol>();
             for (const symbol of symbols) {
                 result[symbol.name] = mappingThisOnly && isIndependentMember(symbol) ? symbol : instantiateSymbol(symbol, mapper);
             }
@@ -3977,7 +3973,7 @@ namespace ts {
 
         function addInheritedMembers(symbols: SymbolTable, baseSymbols: Symbol[]) {
             for (const s of baseSymbols) {
-                if (!hasProperty(symbols, s.name)) {
+                if (!symbols[s.name]) {
                     symbols[s.name] = s;
                 }
             }
@@ -4000,6 +3996,9 @@ namespace ts {
                 return createTypeReference((<TypeReference>type).target,
                     concatenate((<TypeReference>type).typeArguments, [thisArgument || (<TypeReference>type).target.thisType]));
             }
+            if (type.flags & TypeFlags.Tuple) {
+                return createTupleType((type as TupleType).elementTypes, thisArgument);
+            }
             return type;
         }
 
@@ -4091,7 +4090,7 @@ namespace ts {
         }
 
         function createTupleTypeMemberSymbols(memberTypes: Type[]): SymbolTable {
-            const members: SymbolTable = {};
+            const members = createMap<Symbol>();
             for (let i = 0; i < memberTypes.length; i++) {
                 const symbol = <TransientSymbol>createSymbol(SymbolFlags.Property | SymbolFlags.Transient, "" + i);
                 symbol.type = memberTypes[i];
@@ -4103,7 +4102,8 @@ namespace ts {
         function resolveTupleTypeMembers(type: TupleType) {
             const arrayElementType = getUnionType(type.elementTypes);
             // Make the tuple type itself the 'this' type by including an extra type argument
-            const arrayType = resolveStructuredTypeMembers(createTypeFromGenericGlobalType(globalArrayType, [arrayElementType, type]));
+            // (Unless it's provided in the case that the tuple is a type parameter constraint)
+            const arrayType = resolveStructuredTypeMembers(createTypeFromGenericGlobalType(globalArrayType, [arrayElementType, type.thisType || type]));
             const members = createTupleTypeMemberSymbols(type.elementTypes);
             addInheritedMembers(members, arrayType.properties);
             setObjectTypeMembers(type, members, arrayType.callSignatures, arrayType.constructSignatures, arrayType.stringIndexInfo, arrayType.numberIndexInfo);
@@ -4313,11 +4313,9 @@ namespace ts {
         function getPropertyOfObjectType(type: Type, name: string): Symbol {
             if (type.flags & TypeFlags.ObjectType) {
                 const resolved = resolveStructuredTypeMembers(<ObjectType>type);
-                if (hasProperty(resolved.members, name)) {
-                    const symbol = resolved.members[name];
-                    if (symbolIsValue(symbol)) {
-                        return symbol;
-                    }
+                const symbol = resolved.members[name];
+                if (symbol && symbolIsValue(symbol)) {
+                    return symbol;
                 }
             }
         }
@@ -4416,10 +4414,19 @@ namespace ts {
             }
             const propTypes: Type[] = [];
             const declarations: Declaration[] = [];
+            let commonType: Type = undefined;
+            let hasCommonType = true;
             for (const prop of props) {
                 if (prop.declarations) {
                     addRange(declarations, prop.declarations);
                 }
+                const type = getTypeOfSymbol(prop);
+                if (!commonType) {
+                    commonType = type;
+                }
+                else if (type !== commonType) {
+                    hasCommonType = false;
+                }
                 propTypes.push(getTypeOfSymbol(prop));
             }
             const result = <TransientSymbol>createSymbol(
@@ -4429,6 +4436,7 @@ namespace ts {
                 commonFlags,
                 name);
             result.containingType = containingType;
+            result.hasCommonType = hasCommonType;
             result.declarations = declarations;
             result.isReadonly = isReadonly;
             result.type = containingType.flags & TypeFlags.Union ? getUnionType(propTypes) : getIntersectionType(propTypes);
@@ -4436,29 +4444,32 @@ namespace ts {
         }
 
         function getPropertyOfUnionOrIntersectionType(type: UnionOrIntersectionType, name: string): Symbol {
-            const properties = type.resolvedProperties || (type.resolvedProperties = {});
-            if (hasProperty(properties, name)) {
-                return properties[name];
-            }
-            const property = createUnionOrIntersectionProperty(type, name);
-            if (property) {
-                properties[name] = property;
+            const properties = type.resolvedProperties || (type.resolvedProperties = createMap<Symbol>());
+            let property = properties[name];
+            if (!property) {
+                property = createUnionOrIntersectionProperty(type, name);
+                if (property) {
+                    properties[name] = property;
+                }
             }
             return property;
         }
 
-        // Return the symbol for the property with the given name in the given type. Creates synthetic union properties when
-        // necessary, maps primitive types and type parameters are to their apparent types, and augments with properties from
-        // Object and Function as appropriate.
+        /**
+         * Return the symbol for the property with the given name in the given type. Creates synthetic union properties when
+         * necessary, maps primitive types and type parameters are to their apparent types, and augments with properties from
+         * Object and Function as appropriate.
+         *
+         * @param type a type to look up property from
+         * @param name a name of property to look up in a given type
+         */
         function getPropertyOfType(type: Type, name: string): Symbol {
             type = getApparentType(type);
             if (type.flags & TypeFlags.ObjectType) {
                 const resolved = resolveStructuredTypeMembers(type);
-                if (hasProperty(resolved.members, name)) {
-                    const symbol = resolved.members[name];
-                    if (symbolIsValue(symbol)) {
-                        return symbol;
-                    }
+                const symbol = resolved.members[name];
+                if (symbol && symbolIsValue(symbol)) {
+                    return symbol;
                 }
                 if (resolved === anyFunctionType || resolved.callSignatures.length || resolved.constructSignatures.length) {
                     const symbol = getPropertyOfObjectType(globalFunctionType, name);
@@ -4926,24 +4937,27 @@ namespace ts {
         }
 
         function getTypeListId(types: Type[]) {
+            let result = "";
             if (types) {
-                switch (types.length) {
-                    case 1:
-                        return "" + types[0].id;
-                    case 2:
-                        return types[0].id + "," + types[1].id;
-                    default:
-                        let result = "";
-                        for (let i = 0; i < types.length; i++) {
-                            if (i > 0) {
-                                result += ",";
-                            }
-                            result += types[i].id;
-                        }
-                        return result;
+                const length = types.length;
+                let i = 0;
+                while (i < length) {
+                    const startId = types[i].id;
+                    let count = 1;
+                    while (i + count < length && types[i + count].id === startId + count) {
+                        count++;
+                    }
+                    if (result.length) {
+                        result += ",";
+                    }
+                    result += startId;
+                    if (count > 1) {
+                        result += ":" + count;
+                    }
+                    i += count;
                 }
             }
-            return "";
+            return result;
         }
 
         // This function is used to propagate certain flags when creating new object type references and union types.
@@ -5026,7 +5040,7 @@ namespace ts {
             return getDeclaredTypeOfSymbol(symbol);
         }
 
-        function getTypeReferenceName(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference): LeftHandSideExpression | EntityName {
+        function getTypeReferenceName(node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference): EntityNameOrEntityNameExpression | undefined {
             switch (node.kind) {
                 case SyntaxKind.TypeReference:
                     return (<TypeReferenceNode>node).typeName;
@@ -5035,8 +5049,9 @@ namespace ts {
                 case SyntaxKind.ExpressionWithTypeArguments:
                     // We only support expressions that are simple qualified names. For other
                     // expressions this produces undefined.
-                    if (isSupportedExpressionWithTypeArguments(<ExpressionWithTypeArguments>node)) {
-                        return (<ExpressionWithTypeArguments>node).expression;
+                    const expr = (<ExpressionWithTypeArguments>node).expression;
+                    if (isEntityNameExpression(expr)) {
+                        return expr;
                     }
 
                 // fall through;
@@ -5047,7 +5062,7 @@ namespace ts {
 
         function resolveTypeReferenceName(
             node: TypeReferenceNode | ExpressionWithTypeArguments | JSDocTypeReference,
-            typeReferenceName: LeftHandSideExpression | EntityName) {
+            typeReferenceName: EntityNameExpression | EntityName) {
 
             if (!typeReferenceName) {
                 return unknownSymbol;
@@ -5088,15 +5103,14 @@ namespace ts {
                     const typeReferenceName = getTypeReferenceName(node);
                     symbol = resolveTypeReferenceName(node, typeReferenceName);
                     type = getTypeReferenceType(node, symbol);
-
-                    links.resolvedSymbol = symbol;
-                    links.resolvedType = type;
                 }
                 else {
                     // We only support expressions that are simple qualified names. For other expressions this produces undefined.
-                    const typeNameOrExpression = node.kind === SyntaxKind.TypeReference ? (<TypeReferenceNode>node).typeName :
-                        isSupportedExpressionWithTypeArguments(<ExpressionWithTypeArguments>node) ? (<ExpressionWithTypeArguments>node).expression :
-                            undefined;
+                    const typeNameOrExpression: EntityNameOrEntityNameExpression = node.kind === SyntaxKind.TypeReference
+                        ? (<TypeReferenceNode>node).typeName
+                        : isEntityNameExpression((<ExpressionWithTypeArguments>node).expression)
+                            ? <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression
+                            : undefined;
                     symbol = typeNameOrExpression && resolveEntityName(typeNameOrExpression, SymbolFlags.Type) || unknownSymbol;
                     type = symbol === unknownSymbol ? unknownType :
                         symbol.flags & (SymbolFlags.Class | SymbolFlags.Interface) ? getTypeFromClassOrInterfaceReference(node, symbol) :
@@ -5215,15 +5229,16 @@ namespace ts {
             return links.resolvedType;
         }
 
-        function createTupleType(elementTypes: Type[]) {
-            const id = getTypeListId(elementTypes);
-            return tupleTypes[id] || (tupleTypes[id] = createNewTupleType(elementTypes));
+        function createTupleType(elementTypes: Type[], thisType?: Type) {
+            const id = getTypeListId(elementTypes) + "," + (thisType ? thisType.id : 0);
+            return tupleTypes[id] || (tupleTypes[id] = createNewTupleType(elementTypes, thisType));
         }
 
-        function createNewTupleType(elementTypes: Type[]) {
+        function createNewTupleType(elementTypes: Type[], thisType?: Type) {
             const propagatedFlags = getPropagatingFlagsOfTypes(elementTypes, /*excludeKinds*/ 0);
             const type = <TupleType>createObjectType(TypeFlags.Tuple | propagatedFlags);
             type.elementTypes = elementTypes;
+            type.thisType = thisType;
             return type;
         }
 
@@ -5325,12 +5340,12 @@ namespace ts {
             }
         }
 
-        // We deduplicate the constituent types based on object identity. If the subtypeReduction flag is
-        // specified we also reduce the constituent type set to only include types that aren't subtypes of
-        // other types. Subtype reduction is expensive for large union types and is possible only when union
+        // We sort and deduplicate the constituent types based on object identity. If the subtypeReduction
+        // flag is specified we also reduce the constituent type set to only include types that aren't subtypes
+        // of other types. Subtype reduction is expensive for large union types and is possible only when union
         // types are known not to circularly reference themselves (as is the case with union types created by
         // expression constructs such as array literals and the || and ?: operators). Named types can
-        // circularly reference themselves and therefore cannot be deduplicated during their declaration.
+        // circularly reference themselves and therefore cannot be subtype reduced during their declaration.
         // For example, "type Item = string | (() => Item" is a named type that circularly references itself.
         function getUnionType(types: Type[], subtypeReduction?: boolean, aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
             if (types.length === 0) {
@@ -5352,15 +5367,23 @@ namespace ts {
                     typeSet.containsUndefined ? typeSet.containsNonWideningType ? undefinedType : undefinedWideningType :
                         neverType;
             }
-            else if (typeSet.length === 1) {
-                return typeSet[0];
+            return getUnionTypeFromSortedList(typeSet, aliasSymbol, aliasTypeArguments);
+        }
+
+        // This function assumes the constituent type list is sorted and deduplicated.
+        function getUnionTypeFromSortedList(types: Type[], aliasSymbol?: Symbol, aliasTypeArguments?: Type[]): Type {
+            if (types.length === 0) {
+                return neverType;
             }
-            const id = getTypeListId(typeSet);
+            if (types.length === 1) {
+                return types[0];
+            }
+            const id = getTypeListId(types);
             let type = unionTypes[id];
             if (!type) {
-                const propagatedFlags = getPropagatingFlagsOfTypes(typeSet, /*excludeKinds*/ TypeFlags.Nullable);
+                const propagatedFlags = getPropagatingFlagsOfTypes(types, /*excludeKinds*/ TypeFlags.Nullable);
                 type = unionTypes[id] = <UnionType>createObjectType(TypeFlags.Union | propagatedFlags);
-                type.types = typeSet;
+                type.types = types;
                 type.aliasSymbol = aliasSymbol;
                 type.aliasTypeArguments = aliasTypeArguments;
             }
@@ -5452,7 +5475,7 @@ namespace ts {
 
         function getLiteralTypeForText(flags: TypeFlags, text: string) {
             const map = flags & TypeFlags.StringLiteral ? stringLiteralTypes : numericLiteralTypes;
-            return hasProperty(map, text) ? map[text] : map[text] = createLiteralType(flags, text);
+            return map[text] || (map[text] = createLiteralType(flags, text));
         }
 
         function getTypeFromLiteralTypeNode(node: LiteralTypeNode): Type {
@@ -5909,6 +5932,13 @@ namespace ts {
             return isTypeRelatedTo(source, target, assignableRelation);
         }
 
+        // A type S is considered to be an instance of a type T if S and T are the same type or if S is a
+        // subtype of T but not structurally identical to T. This specifically means that two distinct but
+        // structurally identical types (such as two classes) are not considered instances of each other.
+        function isTypeInstanceOf(source: Type, target: Type): boolean {
+            return source === target || isTypeSubtypeOf(source, target) && !isTypeIdenticalTo(source, target);
+        }
+
         /**
          * This is *not* a bi-directional relationship.
          * If one needs to check both directions for comparability, use a second call to this function or 'checkTypeComparableTo'.
@@ -6560,7 +6590,7 @@ namespace ts {
                 }
                 sourceStack[depth] = source;
                 targetStack[depth] = target;
-                maybeStack[depth] = {};
+                maybeStack[depth] = createMap<RelationComparisonResult>();
                 maybeStack[depth][id] = RelationComparisonResult.Succeeded;
                 depth++;
                 const saveExpandingFlags = expandingFlags;
@@ -6591,7 +6621,7 @@ namespace ts {
                     const maybeCache = maybeStack[depth];
                     // If result is definitely true, copy assumptions to global cache, else copy to next level up
                     const destinationCache = (result === Ternary.True || depth === 0) ? relation : maybeStack[depth - 1];
-                    copyMap(maybeCache, destinationCache);
+                    copyProperties(maybeCache, destinationCache);
                 }
                 else {
                     // A false result goes straight into global cache (when something is false under assumptions it
@@ -7201,7 +7231,7 @@ namespace ts {
         }
 
         function transformTypeOfMembers(type: Type, f: (propertyType: Type) => Type) {
-            const members: SymbolTable = {};
+            const members = createMap<Symbol>();
             for (const property of getPropertiesOfObjectType(type)) {
                 const original = getTypeOfSymbol(property);
                 const updated = f(original);
@@ -7425,7 +7455,7 @@ namespace ts {
             let targetStack: Type[];
             let depth = 0;
             let inferiority = 0;
-            const visited: Map<boolean> = {};
+            const visited = createMap<boolean>();
             inferFromTypes(source, target);
 
             function isInProcess(source: Type, target: Type) {
@@ -7559,7 +7589,7 @@ namespace ts {
                             return;
                         }
                         const key = source.id + "," + target.id;
-                        if (hasProperty(visited, key)) {
+                        if (visited[key]) {
                             return;
                         }
                         visited[key] = true;
@@ -7793,8 +7823,49 @@ namespace ts {
             return false;
         }
 
-        function rootContainsMatchingReference(source: Node, target: Node) {
-            return target.kind === SyntaxKind.PropertyAccessExpression && containsMatchingReference(source, (<PropertyAccessExpression>target).expression);
+        // Return true if target is a property access xxx.yyy, source is a property access xxx.zzz, the declared
+        // type of xxx is a union type, and yyy is a property that is possibly a discriminant. We consider a property
+        // a possible discriminant if its type differs in the constituents of containing union type, and if every
+        // choice is a unit type or a union of unit types.
+        function containsMatchingReferenceDiscriminant(source: Node, target: Node) {
+            return target.kind === SyntaxKind.PropertyAccessExpression &&
+                containsMatchingReference(source, (<PropertyAccessExpression>target).expression) &&
+                isDiscriminantProperty(getDeclaredTypeOfReference((<PropertyAccessExpression>target).expression), (<PropertyAccessExpression>target).name.text);
+        }
+
+        function getDeclaredTypeOfReference(expr: Node): Type {
+            if (expr.kind === SyntaxKind.Identifier) {
+                return getTypeOfSymbol(getResolvedSymbol(<Identifier>expr));
+            }
+            if (expr.kind === SyntaxKind.PropertyAccessExpression) {
+                const type = getDeclaredTypeOfReference((<PropertyAccessExpression>expr).expression);
+                return type && getTypeOfPropertyOfType(type, (<PropertyAccessExpression>expr).name.text);
+            }
+            return undefined;
+        }
+
+        function isDiscriminantProperty(type: Type, name: string) {
+            if (type && type.flags & TypeFlags.Union) {
+                let prop = getPropertyOfType(type, name);
+                if (!prop) {
+                    // The type may be a union that includes nullable or primitive types. If filtering
+                    // those out produces a different type, get the property from that type instead.
+                    // Effectively, we're checking if this *could* be a discriminant property once nullable
+                    // and primitive types are removed by other type guards.
+                    const filteredType = getTypeWithFacts(type, TypeFacts.Discriminatable);
+                    if (filteredType !== type && filteredType.flags & TypeFlags.Union) {
+                        prop = getPropertyOfType(filteredType, name);
+                    }
+                }
+                if (prop && prop.flags & SymbolFlags.SyntheticProperty) {
+                    if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
+                        (<TransientSymbol>prop).isDiscriminantProperty = !(<TransientSymbol>prop).hasCommonType &&
+                            isUnitUnionType(getTypeOfSymbol(prop));
+                    }
+                    return (<TransientSymbol>prop).isDiscriminantProperty;
+                }
+            }
+            return false;
         }
 
         function isOrContainsMatchingReference(source: Node, target: Node) {
@@ -7840,10 +7911,10 @@ namespace ts {
         // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
         // we remove type string.
         function getAssignmentReducedType(declaredType: UnionType, assignedType: Type) {
-            if (declaredType !== assignedType && declaredType.flags & TypeFlags.Union) {
-                const reducedTypes = filter(declaredType.types, t => typeMaybeAssignableTo(assignedType, t));
-                if (reducedTypes.length) {
-                    return reducedTypes.length === 1 ? reducedTypes[0] : getUnionType(reducedTypes);
+            if (declaredType !== assignedType) {
+                const reducedType = filterType(declaredType, t => typeMaybeAssignableTo(assignedType, t));
+                if (reducedType !== neverType) {
+                    return reducedType;
                 }
             }
             return declaredType;
@@ -7857,6 +7928,14 @@ namespace ts {
             return result;
         }
 
+        function isFunctionObjectType(type: ObjectType): boolean {
+            // We do a quick check for a "bind" property before performing the more expensive subtype
+            // check. This gives us a quicker out in the common case where an object type is not a function.
+            const resolved = resolveStructuredTypeMembers(type);
+            return !!(resolved.callSignatures.length || resolved.constructSignatures.length ||
+                resolved.members["bind"] && isTypeSubtypeOf(type, globalFunctionType));
+        }
+
         function getTypeFacts(type: Type): TypeFacts {
             const flags = type.flags;
             if (flags & TypeFlags.String) {
@@ -7885,8 +7964,7 @@ namespace ts {
                     type === falseType ? TypeFacts.FalseFacts : TypeFacts.TrueFacts;
             }
             if (flags & TypeFlags.ObjectType) {
-                const resolved = resolveStructuredTypeMembers(type);
-                return resolved.callSignatures.length || resolved.constructSignatures.length || isTypeSubtypeOf(type, globalFunctionType) ?
+                return isFunctionObjectType(<ObjectType>type) ?
                     strictNullChecks ? TypeFacts.FunctionStrictFacts : TypeFacts.FunctionFacts :
                     strictNullChecks ? TypeFacts.ObjectStrictFacts : TypeFacts.ObjectFacts;
             }
@@ -7901,7 +7979,7 @@ namespace ts {
             }
             if (flags & TypeFlags.TypeParameter) {
                 const constraint = getConstraintOfTypeParameter(<TypeParameter>type);
-                return constraint ? getTypeFacts(constraint) : TypeFacts.All;
+                return getTypeFacts(constraint || emptyObjectType);
             }
             if (flags & TypeFlags.UnionOrIntersection) {
                 return getTypeFactsOfTypes((<UnionOrIntersectionType>type).types);
@@ -7910,25 +7988,7 @@ namespace ts {
         }
 
         function getTypeWithFacts(type: Type, include: TypeFacts) {
-            if (!(type.flags & TypeFlags.Union)) {
-                return getTypeFacts(type) & include ? type : neverType;
-            }
-            let firstType: Type;
-            let types: Type[];
-            for (const t of (type as UnionType).types) {
-                if (getTypeFacts(t) & include) {
-                    if (!firstType) {
-                        firstType = t;
-                    }
-                    else {
-                        if (!types) {
-                            types = [firstType];
-                        }
-                        types.push(t);
-                    }
-                }
-            }
-            return firstType ? types ? getUnionType(types) : firstType : neverType;
+            return filterType(type, t => (getTypeFacts(t) & include) !== 0);
         }
 
         function getTypeWithDefault(type: Type, defaultExpression: Expression) {
@@ -8084,27 +8144,61 @@ namespace ts {
             return source.flags & TypeFlags.Union ? !forEach((<UnionType>source).types, t => !contains(types, t)) : contains(types, source);
         }
 
-        function filterType(type: Type, f: (t: Type) => boolean): Type {
-            return type.flags & TypeFlags.Union ?
-                getUnionType(filter((<UnionType>type).types, f)) :
-                f(type) ? type : neverType;
+        function isTypeSubsetOf(source: Type, target: Type) {
+            return source === target || target.flags & TypeFlags.Union && isTypeSubsetOfUnion(source, <UnionType>target);
         }
 
-        function getFlowTypeOfReference(reference: Node, declaredType: Type, assumeInitialized: boolean, includeOuterFunctions: boolean) {
+        function isTypeSubsetOfUnion(source: Type, target: UnionType) {
+            if (source.flags & TypeFlags.Union) {
+                for (const t of (<UnionType>source).types) {
+                    if (!containsType(target.types, t)) {
+                        return false;
+                    }
+                }
+                return true;
+            }
+            if (source.flags & TypeFlags.EnumLiteral && target.flags & TypeFlags.Enum && (<EnumLiteralType>source).baseType === target) {
+                return true;
+            }
+            return containsType(target.types, source);
+        }
+
+        function filterType(type: Type, f: (t: Type) => boolean): Type {
+            if (type.flags & TypeFlags.Union) {
+                const types = (<UnionType>type).types;
+                const filtered = filter(types, f);
+                return filtered === types ? type : getUnionTypeFromSortedList(filtered);
+            }
+            return f(type) ? type : neverType;
+        }
+
+        function isIncomplete(flowType: FlowType) {
+            return flowType.flags === 0;
+        }
+
+        function getTypeFromFlowType(flowType: FlowType) {
+            return flowType.flags === 0 ? (<IncompleteType>flowType).type : <Type>flowType;
+        }
+
+        function createFlowType(type: Type, incomplete: boolean): FlowType {
+            return incomplete ? { flags: 0, type } : type;
+        }
+
+        function getFlowTypeOfReference(reference: Node, declaredType: Type, assumeInitialized: boolean, flowContainer: Node) {
             let key: string;
             if (!reference.flowNode || assumeInitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                 return declaredType;
             }
             const initialType = assumeInitialized ? declaredType : includeFalsyTypes(declaredType, TypeFlags.Undefined);
             const visitedFlowStart = visitedFlowCount;
-            const result = getTypeAtFlowNode(reference.flowNode);
+            const result = getTypeFromFlowType(getTypeAtFlowNode(reference.flowNode));
             visitedFlowCount = visitedFlowStart;
             if (reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(result, TypeFacts.NEUndefinedOrNull) === neverType) {
                 return declaredType;
             }
             return result;
 
-            function getTypeAtFlowNode(flow: FlowNode): Type {
+            function getTypeAtFlowNode(flow: FlowNode): FlowType {
                 while (true) {
                     if (flow.flags & FlowFlags.Shared) {
                         // We cache results of flow type resolution for shared nodes that were previously visited in
@@ -8116,7 +8210,7 @@ namespace ts {
                             }
                         }
                     }
-                    let type: Type;
+                    let type: FlowType;
                     if (flow.flags & FlowFlags.Assignment) {
                         type = getTypeAtFlowAssignment(<FlowAssignment>flow);
                         if (!type) {
@@ -8142,7 +8236,7 @@ namespace ts {
                     else if (flow.flags & FlowFlags.Start) {
                         // Check if we should continue with the control flow of the containing function.
                         const container = (<FlowStart>flow).container;
-                        if (container && includeOuterFunctions) {
+                        if (container && container !== flowContainer && reference.kind !== SyntaxKind.PropertyAccessExpression) {
                             flow = container.flowNode;
                             continue;
                         }
@@ -8184,41 +8278,45 @@ namespace ts {
                 return undefined;
             }
 
-            function getTypeAtFlowCondition(flow: FlowCondition) {
-                let type = getTypeAtFlowNode(flow.antecedent);
+            function getTypeAtFlowCondition(flow: FlowCondition): FlowType {
+                const flowType = getTypeAtFlowNode(flow.antecedent);
+                let type = getTypeFromFlowType(flowType);
                 if (type !== neverType) {
                     // If we have an antecedent type (meaning we're reachable in some way), we first
-                    // attempt to narrow the antecedent type. If that produces the nothing type, then
-                    // we take the type guard as an indication that control could reach here in a
-                    // manner not understood by the control flow analyzer (e.g. a function argument
-                    // has an invalid type, or a nested function has possibly made an assignment to a
-                    // captured variable). We proceed by reverting to the declared type and then
+                    // attempt to narrow the antecedent type. If that produces the never type, and if
+                    // the antecedent type is incomplete (i.e. a transient type in a loop), then we
+                    // take the type guard as an indication that control *could* reach here once we
+                    // have the complete type. We proceed by reverting to the declared type and then
                     // narrow that.
                     const assumeTrue = (flow.flags & FlowFlags.TrueCondition) !== 0;
                     type = narrowType(type, flow.expression, assumeTrue);
-                    if (type === neverType) {
+                    if (type === neverType && isIncomplete(flowType)) {
                         type = narrowType(declaredType, flow.expression, assumeTrue);
                     }
                 }
-                return type;
+                return createFlowType(type, isIncomplete(flowType));
             }
 
-            function getTypeAtSwitchClause(flow: FlowSwitchClause) {
-                const type = getTypeAtFlowNode(flow.antecedent);
+            function getTypeAtSwitchClause(flow: FlowSwitchClause): FlowType {
+                const flowType = getTypeAtFlowNode(flow.antecedent);
+                let type = getTypeFromFlowType(flowType);
                 const expr = flow.switchStatement.expression;
                 if (isMatchingReference(reference, expr)) {
-                    return narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
+                    type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                 }
-                if (isMatchingPropertyAccess(expr)) {
-                    return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
+                else if (isMatchingReferenceDiscriminant(expr)) {
+                    type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                 }
-                return type;
+                return createFlowType(type, isIncomplete(flowType));
             }
 
-            function getTypeAtFlowBranchLabel(flow: FlowLabel) {
+            function getTypeAtFlowBranchLabel(flow: FlowLabel): FlowType {
                 const antecedentTypes: Type[] = [];
+                let subtypeReduction = false;
+                let seenIncomplete = false;
                 for (const antecedent of flow.antecedents) {
-                    const type = getTypeAtFlowNode(antecedent);
+                    const flowType = getTypeAtFlowNode(antecedent);
+                    const type = getTypeFromFlowType(flowType);
                     // If the type at a particular antecedent path is the declared type and the
                     // reference is known to always be assigned (i.e. when declared and initial types
                     // are the same), there is no reason to process more antecedents since the only
@@ -8229,15 +8327,24 @@ namespace ts {
                     if (!contains(antecedentTypes, type)) {
                         antecedentTypes.push(type);
                     }
+                    // If an antecedent type is not a subset of the declared type, we need to perform
+                    // subtype reduction. This happens when a "foreign" type is injected into the control
+                    // flow using the instanceof operator or a user defined type predicate.
+                    if (!isTypeSubsetOf(type, declaredType)) {
+                        subtypeReduction = true;
+                    }
+                    if (isIncomplete(flowType)) {
+                        seenIncomplete = true;
+                    }
                 }
-                return getUnionType(antecedentTypes);
+                return createFlowType(getUnionType(antecedentTypes, subtypeReduction), seenIncomplete);
             }
 
-            function getTypeAtFlowLoopLabel(flow: FlowLabel) {
+            function getTypeAtFlowLoopLabel(flow: FlowLabel): FlowType {
                 // If we have previously computed the control flow type for the reference at
                 // this flow loop junction, return the cached type.
                 const id = getFlowNodeId(flow);
-                const cache = flowLoopCaches[id] || (flowLoopCaches[id] = {});
+                const cache = flowLoopCaches[id] || (flowLoopCaches[id] = createMap<Type>());
                 if (!key) {
                     key = getFlowCacheKey(reference);
                 }
@@ -8245,23 +8352,24 @@ namespace ts {
                     return cache[key];
                 }
                 // If this flow loop junction and reference are already being processed, return
-                // the union of the types computed for each branch so far. We should never see
-                // an empty array here because the first antecedent of a loop junction is always
-                // the non-looping control flow path that leads to the top.
+                // the union of the types computed for each branch so far, marked as incomplete.
+                // We should never see an empty array here because the first antecedent of a loop
+                // junction is always the non-looping control flow path that leads to the top.
                 for (let i = flowLoopStart; i < flowLoopCount; i++) {
                     if (flowLoopNodes[i] === flow && flowLoopKeys[i] === key) {
-                        return getUnionType(flowLoopTypes[i]);
+                        return createFlowType(getUnionType(flowLoopTypes[i]), /*incomplete*/ true);
                     }
                 }
                 // Add the flow loop junction and reference to the in-process stack and analyze
                 // each antecedent code path.
                 const antecedentTypes: Type[] = [];
+                let subtypeReduction = false;
                 flowLoopNodes[flowLoopCount] = flow;
                 flowLoopKeys[flowLoopCount] = key;
                 flowLoopTypes[flowLoopCount] = antecedentTypes;
                 for (const antecedent of flow.antecedents) {
                     flowLoopCount++;
-                    const type = getTypeAtFlowNode(antecedent);
+                    const type = getTypeFromFlowType(getTypeAtFlowNode(antecedent));
                     flowLoopCount--;
                     // If we see a value appear in the cache it is a sign that control flow  analysis
                     // was restarted and completed by checkExpressionCached. We can simply pick up
@@ -8272,6 +8380,12 @@ namespace ts {
                     if (!contains(antecedentTypes, type)) {
                         antecedentTypes.push(type);
                     }
+                    // If an antecedent type is not a subset of the declared type, we need to perform
+                    // subtype reduction. This happens when a "foreign" type is injected into the control
+                    // flow using the instanceof operator or a user defined type predicate.
+                    if (!isTypeSubsetOf(type, declaredType)) {
+                        subtypeReduction = true;
+                    }
                     // If the type at a particular antecedent path is the declared type there is no
                     // reason to process more antecedents since the only possible outcome is subtypes
                     // that will be removed in the final union type anyway.
@@ -8279,13 +8393,14 @@ namespace ts {
                         break;
                     }
                 }
-                return cache[key] = getUnionType(antecedentTypes);
+                return cache[key] = getUnionType(antecedentTypes, subtypeReduction);
             }
 
-            function isMatchingPropertyAccess(expr: Expression) {
+            function isMatchingReferenceDiscriminant(expr: Expression) {
                 return expr.kind === SyntaxKind.PropertyAccessExpression &&
+                    declaredType.flags & TypeFlags.Union &&
                     isMatchingReference(reference, (<PropertyAccessExpression>expr).expression) &&
-                    (declaredType.flags & TypeFlags.Union) !== 0;
+                    isDiscriminantProperty(declaredType, (<PropertyAccessExpression>expr).name.text);
             }
 
             function narrowTypeByDiscriminant(type: Type, propAccess: PropertyAccessExpression, narrowType: (t: Type) => Type): Type {
@@ -8299,10 +8414,10 @@ namespace ts {
                 if (isMatchingReference(reference, expr)) {
                     return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                 }
-                if (isMatchingPropertyAccess(expr)) {
+                if (isMatchingReferenceDiscriminant(expr)) {
                     return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                 }
-                if (rootContainsMatchingReference(reference, expr)) {
+                if (containsMatchingReferenceDiscriminant(reference, expr)) {
                     return declaredType;
                 }
                 return type;
@@ -8331,13 +8446,13 @@ namespace ts {
                         if (isMatchingReference(reference, right)) {
                             return narrowTypeByEquality(type, operator, left, assumeTrue);
                         }
-                        if (isMatchingPropertyAccess(left)) {
+                        if (isMatchingReferenceDiscriminant(left)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                         }
-                        if (isMatchingPropertyAccess(right)) {
+                        if (isMatchingReferenceDiscriminant(right)) {
                             return narrowTypeByDiscriminant(type, <PropertyAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                         }
-                        if (rootContainsMatchingReference(reference, left) || rootContainsMatchingReference(reference, right)) {
+                        if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                             return declaredType;
                         }
                         break;
@@ -8392,16 +8507,16 @@ namespace ts {
                 }
                 if (assumeTrue && !(type.flags & TypeFlags.Union)) {
                     // We narrow a non-union type to an exact primitive type if the non-union type
-                    // is a supertype of that primtive type. For example, type 'any' can be narrowed
+                    // is a supertype of that primitive type. For example, type 'any' can be narrowed
                     // to one of the primitive types.
-                    const targetType = getProperty(typeofTypesByName, literal.text);
+                    const targetType = typeofTypesByName[literal.text];
                     if (targetType && isTypeSubtypeOf(targetType, type)) {
                         return targetType;
                     }
                 }
                 const facts = assumeTrue ?
-                    getProperty(typeofEQFacts, literal.text) || TypeFacts.TypeofEQHostObject :
-                    getProperty(typeofNEFacts, literal.text) || TypeFacts.TypeofNEHostObject;
+                    typeofEQFacts[literal.text] || TypeFacts.TypeofEQHostObject :
+                    typeofNEFacts[literal.text] || TypeFacts.TypeofNEHostObject;
                 return getTypeWithFacts(type, facts);
             }
 
@@ -8432,10 +8547,6 @@ namespace ts {
                     }
                     return type;
                 }
-                // We never narrow type any in an instanceof guard
-                if (isTypeAny(type)) {
-                    return type;
-                }
 
                 // Check that right operand is a function type with a prototype property
                 const rightType = checkExpression(expr.right);
@@ -8453,6 +8564,11 @@ namespace ts {
                     }
                 }
 
+                // Don't narrow from 'any' if the target type is exactly 'Object' or 'Function'
+                if (isTypeAny(type) && (targetType === globalObjectType || targetType === globalFunctionType)) {
+                    return type;
+                }
+
                 if (!targetType) {
                     // Target type is type of construct signature
                     let constructSignatures: Signature[];
@@ -8476,29 +8592,30 @@ namespace ts {
 
             function getNarrowedType(type: Type, candidate: Type, assumeTrue: boolean) {
                 if (!assumeTrue) {
-                    return type.flags & TypeFlags.Union ?
-                        getUnionType(filter((<UnionType>type).types, t => !isTypeSubtypeOf(t, candidate))) :
-                        type;
+                    return filterType(type, t => !isTypeInstanceOf(t, candidate));
                 }
-                // If the current type is a union type, remove all constituents that aren't assignable to
+                // If the current type is a union type, remove all constituents that couldn't be instances of
                 // the candidate type. If one or more constituents remain, return a union of those.
                 if (type.flags & TypeFlags.Union) {
-                    const assignableConstituents = filter((<UnionType>type).types, t => isTypeAssignableTo(t, candidate));
-                    if (assignableConstituents.length) {
-                        return getUnionType(assignableConstituents);
+                    const assignableType = filterType(type, t => isTypeInstanceOf(t, candidate));
+                    if (assignableType !== neverType) {
+                        return assignableType;
                     }
                 }
-                // If the candidate type is assignable to the target type, narrow to the candidate type.
-                // Otherwise, if the current type is assignable to the candidate, keep the current type.
-                // Otherwise, the types are completely unrelated, so narrow to the empty type.
+                // If the candidate type is a subtype of the target type, narrow to the candidate type.
+                // Otherwise, if the target type is assignable to the candidate type, keep the target type.
+                // Otherwise, if the candidate type is assignable to the target type, narrow to the candidate
+                // type. Otherwise, the types are completely unrelated, so narrow to an intersection of the
+                // two types.
                 const targetType = type.flags & TypeFlags.TypeParameter ? getApparentType(type) : type;
-                return isTypeAssignableTo(candidate, targetType) ? candidate :
+                return isTypeSubtypeOf(candidate, targetType) ? candidate :
                     isTypeAssignableTo(type, candidate) ? type :
-                        getIntersectionType([type, candidate]);
+                    isTypeAssignableTo(candidate, targetType) ? candidate :
+                    getIntersectionType([type, candidate]);
             }
 
             function narrowTypeByTypePredicate(type: Type, callExpression: CallExpression, assumeTrue: boolean): Type {
-                if (type.flags & TypeFlags.Any || !hasMatchingArgument(callExpression, reference)) {
+                if (!hasMatchingArgument(callExpression, reference)) {
                     return type;
                 }
                 const signature = getResolvedSignature(callExpression);
@@ -8506,6 +8623,12 @@ namespace ts {
                 if (!predicate) {
                     return type;
                 }
+
+                // Don't narrow from 'any' if the predicate type is exactly 'Object' or 'Function'
+                if (isTypeAny(type) && (predicate.type === globalObjectType || predicate.type === globalFunctionType)) {
+                    return type;
+                }
+
                 if (isIdentifierTypePredicate(predicate)) {
                     const predicateArgument = callExpression.arguments[predicate.parameterIndex];
                     if (predicateArgument) {
@@ -8591,21 +8714,52 @@ namespace ts {
         function getControlFlowContainer(node: Node): Node {
             while (true) {
                 node = node.parent;
-                if (isFunctionLike(node) || node.kind === SyntaxKind.ModuleBlock || node.kind === SyntaxKind.SourceFile || node.kind === SyntaxKind.PropertyDeclaration) {
+                if (isFunctionLike(node) && !getImmediatelyInvokedFunctionExpression(node) ||
+                    node.kind === SyntaxKind.ModuleBlock ||
+                    node.kind === SyntaxKind.SourceFile ||
+                    node.kind === SyntaxKind.PropertyDeclaration) {
                     return node;
                 }
             }
         }
 
-        function isDeclarationIncludedInFlow(reference: Node, declaration: Declaration, includeOuterFunctions: boolean) {
-            const declarationContainer = getControlFlowContainer(declaration);
-            let container = getControlFlowContainer(reference);
-            while (container !== declarationContainer &&
-                (container.kind === SyntaxKind.FunctionExpression || container.kind === SyntaxKind.ArrowFunction) &&
-                (includeOuterFunctions || getImmediatelyInvokedFunctionExpression(<FunctionExpression>container))) {
-                container = getControlFlowContainer(container);
+        // Check if a parameter is assigned anywhere within its declaring function.
+        function isParameterAssigned(symbol: Symbol) {
+            const func = <FunctionLikeDeclaration>getRootDeclaration(symbol.valueDeclaration).parent;
+            const links = getNodeLinks(func);
+            if (!(links.flags & NodeCheckFlags.AssignmentsMarked)) {
+                links.flags |= NodeCheckFlags.AssignmentsMarked;
+                if (!hasParentWithAssignmentsMarked(func)) {
+                    markParameterAssignments(func);
+                }
+            }
+            return symbol.isAssigned || false;
+        }
+
+        function hasParentWithAssignmentsMarked(node: Node) {
+            while (true) {
+                node = node.parent;
+                if (!node) {
+                    return false;
+                }
+                if (isFunctionLike(node) && getNodeLinks(node).flags & NodeCheckFlags.AssignmentsMarked) {
+                    return true;
+                }
+            }
+        }
+
+        function markParameterAssignments(node: Node) {
+            if (node.kind === SyntaxKind.Identifier) {
+                if (isAssignmentTarget(node)) {
+                    const symbol = getResolvedSymbol(<Identifier>node);
+                    if (symbol.valueDeclaration && getRootDeclaration(symbol.valueDeclaration).kind === SyntaxKind.Parameter) {
+                        symbol.isAssigned = true;
+                    }
+                }
+            }
+            else {
+                forEachChild(node, markParameterAssignments);
             }
-            return container === declarationContainer;
         }
 
         function checkIdentifier(node: Identifier): Type {
@@ -8660,15 +8814,35 @@ namespace ts {
             checkNestedBlockScopedBinding(node, symbol);
 
             const type = getTypeOfSymbol(localOrExportSymbol);
-            if (!(localOrExportSymbol.flags & SymbolFlags.Variable) || isAssignmentTarget(node)) {
+            const declaration = localOrExportSymbol.valueDeclaration;
+            // We only narrow variables and parameters occurring in a non-assignment position. For all other
+            // entities we simply return the declared type.
+            if (!(localOrExportSymbol.flags & SymbolFlags.Variable) || isAssignmentTarget(node) || !declaration) {
                 return type;
             }
-            const declaration = localOrExportSymbol.valueDeclaration;
-            const includeOuterFunctions = isReadonlySymbol(localOrExportSymbol);
-            const assumeInitialized = !strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || !declaration ||
-                getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
-                !isDeclarationIncludedInFlow(node, declaration, includeOuterFunctions);
-            const flowType = getFlowTypeOfReference(node, type, assumeInitialized, includeOuterFunctions);
+            // The declaration container is the innermost function that encloses the declaration of the variable
+            // or parameter. The flow container is the innermost function starting with which we analyze the control
+            // flow graph to determine the control flow based type.
+            const isParameter = getRootDeclaration(declaration).kind === SyntaxKind.Parameter;
+            const declarationContainer = getControlFlowContainer(declaration);
+            let flowContainer = getControlFlowContainer(node);
+            // When the control flow originates in a function expression or arrow function and we are referencing
+            // a const variable or parameter from an outer function, we extend the origin of the control flow
+            // analysis to include the immediately enclosing function.
+            while (flowContainer !== declarationContainer &&
+                (flowContainer.kind === SyntaxKind.FunctionExpression || flowContainer.kind === SyntaxKind.ArrowFunction) &&
+                (isReadonlySymbol(localOrExportSymbol) || isParameter && !isParameterAssigned(localOrExportSymbol))) {
+                flowContainer = getControlFlowContainer(flowContainer);
+            }
+            // We only look for uninitialized variables in strict null checking mode, and only when we can analyze
+            // the entire control flow graph from the variable's declaration (i.e. when the flow container and
+            // declaration container are the same).
+            const assumeInitialized = !strictNullChecks || (type.flags & TypeFlags.Any) !== 0 || isParameter ||
+                flowContainer !== declarationContainer || isInAmbientContext(declaration);
+            const flowType = getFlowTypeOfReference(node, type, assumeInitialized, flowContainer);
+            // A variable is considered uninitialized when it is possible to analyze the entire control flow graph
+            // from declaration to use, and when the variable's declared type doesn't include undefined but the
+            // control flow based type does include undefined.
             if (!assumeInitialized && !(getFalsyFlags(type) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
                 error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                 // Return the declared type to reduce follow-on errors
@@ -8921,7 +9095,7 @@ namespace ts {
             if (isClassLike(container.parent)) {
                 const symbol = getSymbolOfNode(container.parent);
                 const type = container.flags & NodeFlags.Static ? getTypeOfSymbol(symbol) : (<InterfaceType>getDeclaredTypeOfSymbol(symbol)).thisType;
-                return getFlowTypeOfReference(node, type, /*assumeInitialized*/ true, /*includeOuterFunctions*/ true);
+                return getFlowTypeOfReference(node, type, /*assumeInitialized*/ true, /*flowContainer*/ undefined);
             }
 
             if (isInJavaScriptFile(node)) {
@@ -9332,6 +9506,11 @@ namespace ts {
             const binaryExpression = <BinaryExpression>node.parent;
             const operator = binaryExpression.operatorToken.kind;
             if (operator >= SyntaxKind.FirstAssignment && operator <= SyntaxKind.LastAssignment) {
+                // Don't do this for special property assignments to avoid circularity
+                if (getSpecialPropertyAssignmentKind(binaryExpression) !== SpecialPropertyAssignmentKind.None) {
+                    return undefined;
+                }
+
                 // In an assignment expression, the right operand is contextually typed by the type of the left operand.
                 if (node === binaryExpression.right) {
                     return checkExpression(binaryExpression.left);
@@ -9823,7 +10002,7 @@ namespace ts {
             // Grammar checking
             checkGrammarObjectLiteralExpression(node, inDestructuringPattern);
 
-            const propertiesTable: SymbolTable = {};
+            const propertiesTable = createMap<Symbol>();
             const propertiesArray: Symbol[] = [];
             const contextualType = getApparentTypeOfContextualType(node);
             const contextualTypeHasPattern = contextualType && contextualType.pattern &&
@@ -9914,7 +10093,7 @@ namespace ts {
             // type with those properties for which the binding pattern specifies a default value.
             if (contextualTypeHasPattern) {
                 for (const prop of getPropertiesOfType(contextualType)) {
-                    if (!hasProperty(propertiesTable, prop.name)) {
+                    if (!propertiesTable[prop.name]) {
                         if (!(prop.flags & SymbolFlags.Optional)) {
                             error(prop.valueDeclaration || (<TransientSymbol>prop).bindingElement,
                                 Diagnostics.Initializer_provides_no_value_for_this_binding_element_and_the_binding_element_has_no_default_value);
@@ -10004,10 +10183,9 @@ namespace ts {
                 const correspondingPropSymbol = getPropertyOfType(elementAttributesType, node.name.text);
                 correspondingPropType = correspondingPropSymbol && getTypeOfSymbol(correspondingPropSymbol);
                 if (isUnhyphenatedJsxName(node.name.text)) {
-                    // Maybe there's a string indexer?
-                    const indexerType = getIndexTypeOfType(elementAttributesType, IndexKind.String);
-                    if (indexerType) {
-                        correspondingPropType = indexerType;
+                    const attributeType = getTypeOfPropertyOfType(elementAttributesType, getTextOfPropertyName(node.name)) || getIndexTypeOfType(elementAttributesType, IndexKind.String);
+                    if (attributeType) {
+                        correspondingPropType = attributeType;
                     }
                     else {
                         // If there's no corresponding property with this name, error
@@ -10364,7 +10542,7 @@ namespace ts {
 
             const targetAttributesType = getJsxElementAttributesType(node);
 
-            const nameTable: Map<boolean> = {};
+            const nameTable = createMap<boolean>();
             // Process this array in right-to-left order so we know which
             // attributes (mostly from spreads) are being overwritten and
             // thus should have their types ignored
@@ -10388,7 +10566,7 @@ namespace ts {
                 const targetProperties = getPropertiesOfType(targetAttributesType);
                 for (let i = 0; i < targetProperties.length; i++) {
                     if (!(targetProperties[i].flags & SymbolFlags.Optional) &&
-                        nameTable[targetProperties[i].name] === undefined) {
+                        !nameTable[targetProperties[i].name]) {
 
                         error(node, Diagnostics.Property_0_is_missing_in_type_1, targetProperties[i].name, typeToString(targetAttributesType));
                     }
@@ -10582,7 +10760,7 @@ namespace ts {
                 !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
                 return propType;
             }
-            return getFlowTypeOfReference(node, propType, /*assumeInitialized*/ true, /*includeOuterFunctions*/ false);
+            return getFlowTypeOfReference(node, propType, /*assumeInitialized*/ true, /*flowContainer*/ undefined);
         }
 
         function isValidPropertyAccess(node: PropertyAccessExpression | QualifiedName, propertyName: string): boolean {
@@ -10707,10 +10885,11 @@ namespace ts {
             }
 
             // Check for compatible indexer types.
-            if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbol)) {
+            const allowedNullableFlags = strictNullChecks ? 0 : TypeFlags.Nullable;
+            if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbol | allowedNullableFlags)) {
 
                 // Try to use a number indexer.
-                if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.NumberLike) || isForInVariableForNumericPropertyNames(node.argumentExpression)) {
+                if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.NumberLike | allowedNullableFlags) || isForInVariableForNumericPropertyNames(node.argumentExpression)) {
                     const numberIndexInfo = getIndexInfoOfType(objectType, IndexKind.Number);
                     if (numberIndexInfo) {
                         getNodeLinks(node).resolvedIndexInfo = numberIndexInfo;
@@ -12837,6 +13016,9 @@ namespace ts {
                         return checkDestructuringAssignment(element, type, contextualMapper);
                     }
                     else {
+                        // We still need to check element expression here because we may need to set appropriate flag on the expression
+                        // such as NodeCheckFlags.LexicalThis on "this"expression.
+                        checkExpression(element);
                         if (isTupleType(sourceType)) {
                             error(element, Diagnostics.Tuple_type_0_with_length_1_cannot_be_assigned_to_tuple_with_length_2, typeToString(sourceType), (<TupleType>sourceType).elementTypes.length, elements.length);
                         }
@@ -13667,15 +13849,19 @@ namespace ts {
         }
 
         function checkClassForDuplicateDeclarations(node: ClassLikeDeclaration) {
-            const getter = 1, setter = 2, property = getter | setter;
+            const enum Accessor {
+                Getter = 1,
+                Setter = 2,
+                Property = Getter | Setter
+            }
 
-            const instanceNames: Map<number> = {};
-            const staticNames: Map<number> = {};
+            const instanceNames = createMap<Accessor>();
+            const staticNames = createMap<Accessor>();
             for (const member of node.members) {
                 if (member.kind === SyntaxKind.Constructor) {
                     for (const param of (member as ConstructorDeclaration).parameters) {
                         if (isParameterPropertyDeclaration(param)) {
-                            addName(instanceNames, param.name, (param.name as Identifier).text, property);
+                            addName(instanceNames, param.name, (param.name as Identifier).text, Accessor.Property);
                         }
                     }
                 }
@@ -13687,24 +13873,24 @@ namespace ts {
                     if (memberName) {
                         switch (member.kind) {
                             case SyntaxKind.GetAccessor:
-                                addName(names, member.name, memberName, getter);
+                                addName(names, member.name, memberName, Accessor.Getter);
                                 break;
 
                             case SyntaxKind.SetAccessor:
-                                addName(names, member.name, memberName, setter);
+                                addName(names, member.name, memberName, Accessor.Setter);
                                 break;
 
                             case SyntaxKind.PropertyDeclaration:
-                                addName(names, member.name, memberName, property);
+                                addName(names, member.name, memberName, Accessor.Property);
                                 break;
                         }
                     }
                 }
             }
 
-            function addName(names: Map<number>, location: Node, name: string, meaning: number) {
-                if (hasProperty(names, name)) {
-                    const prev = names[name];
+            function addName(names: Map<Accessor>, location: Node, name: string, meaning: Accessor) {
+                const prev = names[name];
+                if (prev) {
                     if (prev & meaning) {
                         error(location, Diagnostics.Duplicate_identifier_0, getTextOfNode(location));
                     }
@@ -13719,7 +13905,7 @@ namespace ts {
         }
 
         function checkObjectTypeForDuplicateDeclarations(node: TypeLiteralNode | InterfaceDeclaration) {
-            const names: Map<boolean> = {};
+            const names = createMap<boolean>();
             for (const member of node.members) {
                 if (member.kind == SyntaxKind.PropertySignature) {
                     let memberName: string;
@@ -13733,7 +13919,7 @@ namespace ts {
                             continue;
                     }
 
-                    if (hasProperty(names, memberName)) {
+                    if (names[memberName]) {
                         error(member.symbol.valueDeclaration.name, Diagnostics.Duplicate_identifier_0, memberName);
                         error(member.name, Diagnostics.Duplicate_identifier_0, memberName);
                     }
@@ -14945,22 +15131,20 @@ namespace ts {
         function checkUnusedLocalsAndParameters(node: Node): void {
             if (node.parent.kind !== SyntaxKind.InterfaceDeclaration && noUnusedIdentifiers && !isInAmbientContext(node)) {
                 for (const key in node.locals) {
-                    if (hasProperty(node.locals, key)) {
-                        const local = node.locals[key];
-                        if (!local.isReferenced) {
-                            if (local.valueDeclaration && local.valueDeclaration.kind === SyntaxKind.Parameter) {
-                                const parameter = <ParameterDeclaration>local.valueDeclaration;
-                                if (compilerOptions.noUnusedParameters &&
-                                    !isParameterPropertyDeclaration(parameter) &&
-                                    !parameterIsThisKeyword(parameter) &&
-                                    !parameterNameStartsWithUnderscore(parameter)) {
-                                    error(local.valueDeclaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
-                                }
-                            }
-                            else if (compilerOptions.noUnusedLocals) {
-                                forEach(local.declarations, d => error(d.name || d, Diagnostics._0_is_declared_but_never_used, local.name));
+                    const local = node.locals[key];
+                    if (!local.isReferenced) {
+                        if (local.valueDeclaration && local.valueDeclaration.kind === SyntaxKind.Parameter) {
+                            const parameter = <ParameterDeclaration>local.valueDeclaration;
+                            if (compilerOptions.noUnusedParameters &&
+                                !isParameterPropertyDeclaration(parameter) &&
+                                !parameterIsThisKeyword(parameter) &&
+                                !parameterNameStartsWithUnderscore(parameter)) {
+                                error(local.valueDeclaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
                             }
                         }
+                        else if (compilerOptions.noUnusedLocals) {
+                            forEach(local.declarations, d => error(d.name || d, Diagnostics._0_is_declared_but_never_used, local.name));
+                        }
                     }
                 }
             }
@@ -14986,7 +15170,7 @@ namespace ts {
                         else if (member.kind === SyntaxKind.Constructor) {
                             for (const parameter of (<ConstructorDeclaration>member).parameters) {
                                 if (!parameter.symbol.isReferenced && parameter.flags & NodeFlags.Private) {
-                                    error(parameter.name, Diagnostics._0_is_declared_but_never_used, parameter.symbol.name);
+                                    error(parameter.name, Diagnostics.Property_0_is_declared_but_never_used, parameter.symbol.name);
                                 }
                             }
                         }
@@ -15017,13 +15201,11 @@ namespace ts {
         function checkUnusedModuleMembers(node: ModuleDeclaration | SourceFile): void {
             if (compilerOptions.noUnusedLocals && !isInAmbientContext(node)) {
                 for (const key in node.locals) {
-                    if (hasProperty(node.locals, key)) {
-                        const local = node.locals[key];
-                        if (!local.isReferenced && !local.exportSymbol) {
-                            for (const declaration of local.declarations) {
-                                if (!isAmbientModule(declaration)) {
-                                    error(declaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
-                                }
+                    const local = node.locals[key];
+                    if (!local.isReferenced && !local.exportSymbol) {
+                        for (const declaration of local.declarations) {
+                            if (!isAmbientModule(declaration)) {
+                                error(declaration.name, Diagnostics._0_is_declared_but_never_used, local.name);
                             }
                         }
                     }
@@ -16032,7 +16214,7 @@ namespace ts {
                     else {
                         const identifierName = (<Identifier>catchClause.variableDeclaration.name).text;
                         const locals = catchClause.block.locals;
-                        if (locals && hasProperty(locals, identifierName)) {
+                        if (locals) {
                             const localSymbol = locals[identifierName];
                             if (localSymbol && (localSymbol.flags & SymbolFlags.BlockScopedVariable) !== 0) {
                                 grammarErrorOnNode(localSymbol.valueDeclaration, Diagnostics.Cannot_redeclare_identifier_0_in_catch_clause, identifierName);
@@ -16269,7 +16451,7 @@ namespace ts {
             const implementedTypeNodes = getClassImplementsHeritageClauseElements(node);
             if (implementedTypeNodes) {
                 for (const typeRefNode of implementedTypeNodes) {
-                    if (!isSupportedExpressionWithTypeArguments(typeRefNode)) {
+                    if (!isEntityNameExpression(typeRefNode.expression)) {
                         error(typeRefNode.expression, Diagnostics.A_class_can_only_implement_an_identifier_Slashqualified_name_with_optional_type_arguments);
                     }
                     checkTypeReferenceNode(typeRefNode);
@@ -16453,18 +16635,18 @@ namespace ts {
                 return true;
             }
 
-            const seen: Map<{ prop: Symbol; containingType: Type }> = {};
+            const seen = createMap<{ prop: Symbol; containingType: Type }>();
             forEach(resolveDeclaredMembers(type).declaredProperties, p => { seen[p.name] = { prop: p, containingType: type }; });
             let ok = true;
 
             for (const base of baseTypes) {
                 const properties = getPropertiesOfObjectType(getTypeWithThisArgument(base, type.thisType));
                 for (const prop of properties) {
-                    if (!hasProperty(seen, prop.name)) {
+                    const existing = seen[prop.name];
+                    if (!existing) {
                         seen[prop.name] = { prop: prop, containingType: base };
                     }
                     else {
-                        const existing = seen[prop.name];
                         const isInheritedProperty = existing.containingType !== type;
                         if (isInheritedProperty && !isPropertyIdenticalTo(existing.prop, prop)) {
                             ok = false;
@@ -16511,7 +16693,7 @@ namespace ts {
                 checkObjectTypeForDuplicateDeclarations(node);
             }
             forEach(getInterfaceBaseTypeNodes(node), heritageElement => {
-                if (!isSupportedExpressionWithTypeArguments(heritageElement)) {
+                if (!isEntityNameExpression(heritageElement.expression)) {
                     error(heritageElement.expression, Diagnostics.An_interface_can_only_extend_an_identifier_Slashqualified_name_with_optional_type_arguments);
                 }
                 checkTypeReferenceNode(heritageElement);
@@ -16976,20 +17158,21 @@ namespace ts {
             }
         }
 
-        function getFirstIdentifier(node: EntityName | Expression): Identifier {
-            while (true) {
-                if (node.kind === SyntaxKind.QualifiedName) {
-                    node = (<QualifiedName>node).left;
-                }
-                else if (node.kind === SyntaxKind.PropertyAccessExpression) {
-                    node = (<PropertyAccessExpression>node).expression;
-                }
-                else {
-                    break;
-                }
+        function getFirstIdentifier(node: EntityNameOrEntityNameExpression): Identifier {
+            switch (node.kind) {
+                case SyntaxKind.Identifier:
+                    return <Identifier>node;
+                case SyntaxKind.QualifiedName:
+                    do {
+                        node = (<QualifiedName>node).left;
+                    } while (node.kind !== SyntaxKind.Identifier);
+                    return <Identifier>node;
+                case SyntaxKind.PropertyAccessExpression:
+                    do {
+                        node = (<PropertyAccessEntityNameExpression>node).expression;
+                    } while (node.kind !== SyntaxKind.Identifier);
+                    return <Identifier>node;
             }
-            Debug.assert(node.kind === SyntaxKind.Identifier);
-            return <Identifier>node;
         }
 
         function checkExternalImportOrExportDeclaration(node: ImportDeclaration | ImportEqualsDeclaration | ExportDeclaration): boolean {
@@ -17418,11 +17601,10 @@ namespace ts {
         }
 
         function checkSourceFile(node: SourceFile) {
-            const start = performance.mark();
-
+            performance.mark("beforeCheck");
             checkSourceFileWorker(node);
-
-            performance.measure("Check", start);
+            performance.mark("afterCheck");
+            performance.measure("Check", "beforeCheck", "afterCheck");
         }
 
         // Fully type check a source file and collect the relevant diagnostics.
@@ -17522,7 +17704,7 @@ namespace ts {
         }
 
         function getSymbolsInScope(location: Node, meaning: SymbolFlags): Symbol[] {
-            const symbols: SymbolTable = {};
+            const symbols = createMap<Symbol>();
             let memberFlags: NodeFlags = 0;
 
             if (isInsideWithStatementBody(location)) {
@@ -17600,7 +17782,7 @@ namespace ts {
                     // We will copy all symbol regardless of its reserved name because
                     // symbolsToArray will check whether the key is a reserved name and
                     // it will not copy symbol with reserved name to the array
-                    if (!hasProperty(symbols, id)) {
+                    if (!symbols[id]) {
                         symbols[id] = symbol;
                     }
                 }
@@ -17688,7 +17870,7 @@ namespace ts {
             return getLeftSideOfImportEqualsOrExportAssignment(node) !== undefined;
         }
 
-        function getSymbolOfEntityNameOrPropertyAccessExpression(entityName: EntityName | PropertyAccessExpression): Symbol {
+        function getSymbolOfEntityNameOrPropertyAccessExpression(entityName: EntityName | PropertyAccessExpression): Symbol | undefined {
             if (isDeclarationName(entityName)) {
                 return getSymbolOfNode(entityName.parent);
             }
@@ -17707,22 +17889,20 @@ namespace ts {
                 }
             }
 
-            if (entityName.parent.kind === SyntaxKind.ExportAssignment) {
-                return resolveEntityName(<Identifier>entityName,
+            if (entityName.parent.kind === SyntaxKind.ExportAssignment && isEntityNameExpression(<Identifier | PropertyAccessExpression>entityName)) {
+                return resolveEntityName(<EntityNameExpression>entityName,
                     /*all meanings*/ SymbolFlags.Value | SymbolFlags.Type | SymbolFlags.Namespace | SymbolFlags.Alias);
             }
 
-            if (entityName.kind !== SyntaxKind.PropertyAccessExpression) {
-                if (isInRightSideOfImportOrExportAssignment(<EntityName>entityName)) {
-                    // Since we already checked for ExportAssignment, this really could only be an Import
-                    const importEqualsDeclaration = <ImportEqualsDeclaration>getAncestor(entityName, SyntaxKind.ImportEqualsDeclaration);
-                    Debug.assert(importEqualsDeclaration !== undefined);
-                    return getSymbolOfPartOfRightHandSideOfImportEquals(<EntityName>entityName, importEqualsDeclaration, /*dontResolveAlias*/ true);
-                }
+            if (entityName.kind !== SyntaxKind.PropertyAccessExpression && isInRightSideOfImportOrExportAssignment(<EntityName>entityName)) {
+                // Since we already checked for ExportAssignment, this really could only be an Import
+                const importEqualsDeclaration = <ImportEqualsDeclaration>getAncestor(entityName, SyntaxKind.ImportEqualsDeclaration);
+                Debug.assert(importEqualsDeclaration !== undefined);
+                return getSymbolOfPartOfRightHandSideOfImportEquals(<EntityName>entityName, importEqualsDeclaration, /*dontResolveAlias*/ true);
             }
 
             if (isRightSideOfQualifiedNameOrPropertyAccess(entityName)) {
-                entityName = <QualifiedName | PropertyAccessExpression>entityName.parent;
+                entityName = <QualifiedName | PropertyAccessEntityNameExpression>entityName.parent;
             }
 
             if (isHeritageClauseElementIdentifier(<EntityName>entityName)) {
@@ -18021,7 +18201,7 @@ namespace ts {
             const propsByName = createSymbolTable(getPropertiesOfType(type));
             if (getSignaturesOfType(type, SignatureKind.Call).length || getSignaturesOfType(type, SignatureKind.Construct).length) {
                 forEach(getPropertiesOfType(globalFunctionType), p => {
-                    if (!hasProperty(propsByName, p.name)) {
+                    if (!propsByName[p.name]) {
                         propsByName[p.name] = p;
                     }
                 });
@@ -18078,7 +18258,7 @@ namespace ts {
                 // otherwise - check if at least one export is value
                 symbolLinks.exportsSomeValue = hasExportAssignment
                     ? !!(moduleSymbol.flags & SymbolFlags.Value)
-                    : forEachValue(getExportsOfModule(moduleSymbol), isValue);
+                    : forEachProperty(getExportsOfModule(moduleSymbol), isValue);
             }
 
             return symbolLinks.exportsSomeValue;
@@ -18369,7 +18549,7 @@ namespace ts {
         }
 
         function hasGlobalName(name: string): boolean {
-            return hasProperty(globals, name);
+            return !!globals[name];
         }
 
         function getReferencedValueSymbol(reference: Identifier): Symbol {
@@ -18393,9 +18573,6 @@ namespace ts {
                 // populate reverse mapping: file path -> type reference directive that was resolved to this file
                 fileToDirective = createFileMap<string>();
                 for (const key in resolvedTypeReferenceDirectives) {
-                    if (!hasProperty(resolvedTypeReferenceDirectives, key)) {
-                        continue;
-                    }
                     const resolvedDirective = resolvedTypeReferenceDirectives[key];
                     if (!resolvedDirective) {
                         continue;
@@ -18435,7 +18612,7 @@ namespace ts {
             };
 
             // defined here to avoid outer scope pollution
-            function getTypeReferenceDirectivesForEntityName(node: EntityName | PropertyAccessExpression): string[] {
+            function getTypeReferenceDirectivesForEntityName(node: EntityNameOrEntityNameExpression): string[] {
                 // program does not have any files with type reference directives - bail out
                 if (!fileToDirective) {
                     return undefined;
@@ -19184,7 +19361,7 @@ namespace ts {
         }
 
         function checkGrammarObjectLiteralExpression(node: ObjectLiteralExpression, inDestructuring: boolean) {
-            const seen: Map<SymbolFlags> = {};
+            const seen = createMap<SymbolFlags>();
             const Property = 1;
             const GetAccessor = 2;
             const SetAccessor = 4;
@@ -19246,7 +19423,7 @@ namespace ts {
                     continue;
                 }
 
-                if (!hasProperty(seen, effectiveName)) {
+                if (!seen[effectiveName]) {
                     seen[effectiveName] = currentKind;
                 }
                 else {
@@ -19270,7 +19447,7 @@ namespace ts {
         }
 
         function checkGrammarJsxElement(node: JsxOpeningLikeElement) {
-            const seen: Map<boolean> = {};
+            const seen = createMap<boolean>();
             for (const attr of node.attributes) {
                 if (attr.kind === SyntaxKind.JsxSpreadAttribute) {
                     continue;
@@ -19278,7 +19455,7 @@ namespace ts {
 
                 const jsxAttr = (<JsxAttribute>attr);
                 const name = jsxAttr.name;
-                if (!hasProperty(seen, name.text)) {
+                if (!seen[name.text]) {
                     seen[name.text] = true;
                 }
                 else {
@@ -19673,7 +19850,7 @@ namespace ts {
         }
 
         function checkGrammarTopLevelElementForRequiredDeclareModifier(node: Node): boolean {
-            // A declare modifier is required for any top level .d.ts declaration except export=, export default,
+            // A declare modifier is required for any top level .d.ts declaration except export=, export default, export as namespace
             // interfaces and imports categories:
             //
             //  DeclarationElement:
@@ -19691,6 +19868,7 @@ namespace ts {
                 node.kind === SyntaxKind.ImportEqualsDeclaration ||
                 node.kind === SyntaxKind.ExportDeclaration ||
                 node.kind === SyntaxKind.ExportAssignment ||
+                node.kind === SyntaxKind.NamespaceExportDeclaration ||
                 (node.flags & NodeFlags.Ambient) ||
                 (node.flags & (NodeFlags.Export | NodeFlags.Default))) {
 
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 7e2c6eb8d33..616bf5e70c0 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -61,10 +61,10 @@ namespace ts {
         },
         {
             name: "jsx",
-            type: {
+            type: createMap({
                 "preserve": JsxEmit.Preserve,
                 "react": JsxEmit.React
-            },
+            }),
             paramType: Diagnostics.KIND,
             description: Diagnostics.Specify_JSX_code_generation_Colon_preserve_or_react,
         },
@@ -91,7 +91,7 @@ namespace ts {
         {
             name: "module",
             shortName: "m",
-            type: {
+            type: createMap({
                 "none": ModuleKind.None,
                 "commonjs": ModuleKind.CommonJS,
                 "amd": ModuleKind.AMD,
@@ -99,16 +99,16 @@ namespace ts {
                 "umd": ModuleKind.UMD,
                 "es6": ModuleKind.ES6,
                 "es2015": ModuleKind.ES2015,
-            },
+            }),
             description: Diagnostics.Specify_module_code_generation_Colon_commonjs_amd_system_umd_or_es2015,
             paramType: Diagnostics.KIND,
         },
         {
             name: "newLine",
-            type: {
+            type: createMap({
                 "crlf": NewLineKind.CarriageReturnLineFeed,
                 "lf": NewLineKind.LineFeed
-            },
+            }),
             description: Diagnostics.Specify_the_end_of_line_sequence_to_be_used_when_emitting_files_Colon_CRLF_dos_or_LF_unix,
             paramType: Diagnostics.NEWLINE,
         },
@@ -250,12 +250,12 @@ namespace ts {
         {
             name: "target",
             shortName: "t",
-            type: {
+            type: createMap({
                 "es3": ScriptTarget.ES3,
                 "es5": ScriptTarget.ES5,
                 "es6": ScriptTarget.ES6,
                 "es2015": ScriptTarget.ES2015,
-            },
+            }),
             description: Diagnostics.Specify_ECMAScript_target_version_Colon_ES3_default_ES5_or_ES2015,
             paramType: Diagnostics.VERSION,
         },
@@ -284,10 +284,10 @@ namespace ts {
         },
         {
             name: "moduleResolution",
-            type: {
+            type: createMap({
                 "node": ModuleResolutionKind.NodeJs,
                 "classic": ModuleResolutionKind.Classic,
-            },
+            }),
             description: Diagnostics.Specify_module_resolution_strategy_Colon_node_Node_js_or_classic_TypeScript_pre_1_6,
         },
         {
@@ -392,7 +392,7 @@ namespace ts {
             type: "list",
             element: {
                 name: "lib",
-                type: {
+                type: createMap({
                     // JavaScript only
                     "es5": "lib.es5.d.ts",
                     "es6": "lib.es2015.d.ts",
@@ -417,7 +417,7 @@ namespace ts {
                     "es2016.array.include": "lib.es2016.array.include.d.ts",
                     "es2017.object": "lib.es2017.object.d.ts",
                     "es2017.sharedmemory": "lib.es2017.sharedmemory.d.ts"
-                },
+                }),
             },
             description: Diagnostics.Specify_library_files_to_be_included_in_the_compilation_Colon
         },
@@ -470,8 +470,8 @@ namespace ts {
             return optionNameMapCache;
         }
 
-        const optionNameMap: Map<CommandLineOption> = {};
-        const shortOptionNames: Map<string> = {};
+        const optionNameMap = createMap<CommandLineOption>();
+        const shortOptionNames = createMap<string>();
         forEach(optionDeclarations, option => {
             optionNameMap[option.name.toLowerCase()] = option;
             if (option.shortName) {
@@ -486,10 +486,9 @@ namespace ts {
     /* @internal */
     export function createCompilerDiagnosticForInvalidCustomType(opt: CommandLineOptionOfCustomType): Diagnostic {
         const namesOfType: string[] = [];
-        forEachKey(opt.type, key => {
+        for (const key in opt.type) {
             namesOfType.push(` '${key}'`);
-        });
-
+        }
         return createCompilerDiagnostic(Diagnostics.Argument_for_0_option_must_be_Colon_1, `--${opt.name}`, namesOfType);
     }
 
@@ -497,7 +496,7 @@ namespace ts {
     export function parseCustomTypeOption(opt: CommandLineOptionOfCustomType, value: string, errors: Diagnostic[]) {
         const key = trimString((value || "")).toLowerCase();
         const map = opt.type;
-        if (hasProperty(map, key)) {
+        if (key in map) {
             return map[key];
         }
         else {
@@ -551,11 +550,11 @@ namespace ts {
                     s = s.slice(s.charCodeAt(1) === CharacterCodes.minus ? 2 : 1).toLowerCase();
 
                     // Try to translate short option names to their full equivalents.
-                    if (hasProperty(shortOptionNames, s)) {
+                    if (s in shortOptionNames) {
                         s = shortOptionNames[s];
                     }
 
-                    if (hasProperty(optionNameMap, s)) {
+                    if (s in optionNameMap) {
                         const opt = optionNameMap[s];
 
                         if (opt.isTSConfigOnly) {
@@ -811,7 +810,7 @@ namespace ts {
         const optionNameMap = arrayToMap(optionDeclarations, opt => opt.name);
 
         for (const id in jsonOptions) {
-            if (hasProperty(optionNameMap, id)) {
+            if (id in optionNameMap) {
                 const opt = optionNameMap[id];
                 defaultOptions[opt.name] = convertJsonOption(opt, jsonOptions[id], basePath, errors);
             }
@@ -848,7 +847,7 @@ namespace ts {
 
     function convertJsonOptionOfCustomType(opt: CommandLineOptionOfCustomType, value: string, errors: Diagnostic[]) {
         const key = value.toLowerCase();
-        if (hasProperty(opt.type, key)) {
+        if (key in opt.type) {
             return opt.type[key];
         }
         else {
@@ -958,12 +957,12 @@ namespace ts {
         // Literal file names (provided via the "files" array in tsconfig.json) are stored in a
         // file map with a possibly case insensitive key. We use this map later when when including
         // wildcard paths.
-        const literalFileMap: Map<string> = {};
+        const literalFileMap = createMap<string>();
 
         // Wildcard paths (provided via the "includes" array in tsconfig.json) are stored in a
         // file map with a possibly case insensitive key. We use this map to store paths matched
         // via wildcard, and to handle extension priority.
-        const wildcardFileMap: Map<string> = {};
+        const wildcardFileMap = createMap<string>();
 
         if (include) {
             include = validateSpecs(include, errors, /*allowTrailingRecursion*/ false);
@@ -1011,7 +1010,7 @@ namespace ts {
                 removeWildcardFilesWithLowerPriorityExtension(file, wildcardFileMap, supportedExtensions, keyMapper);
 
                 const key = keyMapper(file);
-                if (!hasProperty(literalFileMap, key) && !hasProperty(wildcardFileMap, key)) {
+                if (!(key in literalFileMap) && !(key in wildcardFileMap)) {
                     wildcardFileMap[key] = file;
                 }
             }
@@ -1063,7 +1062,7 @@ namespace ts {
         //  /a/b/a?z    - Watch /a/b directly to catch any new file matching a?z
         const rawExcludeRegex = getRegularExpressionForWildcard(exclude, path, "exclude");
         const excludeRegex = rawExcludeRegex && new RegExp(rawExcludeRegex, useCaseSensitiveFileNames ? "" : "i");
-        const wildcardDirectories: Map<WatchDirectoryFlags> = {};
+        const wildcardDirectories = createMap<WatchDirectoryFlags>();
         if (include !== undefined) {
             const recursiveKeys: string[] = [];
             for (const file of include) {
@@ -1076,7 +1075,7 @@ namespace ts {
                 if (match) {
                     const key = useCaseSensitiveFileNames ? match[0] : match[0].toLowerCase();
                     const flags = watchRecursivePattern.test(name) ? WatchDirectoryFlags.Recursive : WatchDirectoryFlags.None;
-                    const existingFlags = getProperty(wildcardDirectories, key);
+                    const existingFlags = wildcardDirectories[key];
                     if (existingFlags === undefined || existingFlags < flags) {
                         wildcardDirectories[key] = flags;
                         if (flags === WatchDirectoryFlags.Recursive) {
@@ -1088,11 +1087,9 @@ namespace ts {
 
             // Remove any subpaths under an existing recursively watched directory.
             for (const key in wildcardDirectories) {
-                if (hasProperty(wildcardDirectories, key)) {
-                    for (const recursiveKey of recursiveKeys) {
-                        if (key !== recursiveKey && containsPath(recursiveKey, key, path, !useCaseSensitiveFileNames)) {
-                            delete wildcardDirectories[key];
-                        }
+                for (const recursiveKey of recursiveKeys) {
+                    if (key !== recursiveKey && containsPath(recursiveKey, key, path, !useCaseSensitiveFileNames)) {
+                        delete wildcardDirectories[key];
                     }
                 }
             }
@@ -1115,7 +1112,7 @@ namespace ts {
         for (let i = ExtensionPriority.Highest; i < adjustedExtensionPriority; i++) {
             const higherPriorityExtension = extensions[i];
             const higherPriorityPath = keyMapper(changeExtension(file, higherPriorityExtension));
-            if (hasProperty(literalFiles, higherPriorityPath) || hasProperty(wildcardFiles, higherPriorityPath)) {
+            if (higherPriorityPath in literalFiles || higherPriorityPath in wildcardFiles) {
                 return true;
             }
         }
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 709a331e022..40341929c02 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -19,8 +19,28 @@ namespace ts {
         True = -1
     }
 
+    const createObject = Object.create;
+
+    export function createMap<T>(template?: MapLike<T>): Map<T> {
+        const map: Map<T> = createObject(null); // tslint:disable-line:no-null-keyword
+
+        // Using 'delete' on an object causes V8 to put the object in dictionary mode.
+        // This disables creation of hidden classes, which are expensive when an object is
+        // constantly changing shape.
+        map["__"] = undefined;
+        delete map["__"];
+
+        // Copies keys/values from template. Note that for..in will not throw if
+        // template is undefined, and instead will just exit the loop.
+        for (const key in template) if (hasOwnProperty.call(template, key)) {
+            map[key] = template[key];
+        }
+
+        return map;
+    }
+
     export function createFileMap<T>(keyMapper?: (key: string) => string): FileMap<T> {
-        let files: Map<T> = {};
+        let files = createMap<T>();
         return {
             get,
             set,
@@ -46,7 +66,7 @@ namespace ts {
         }
 
         function contains(path: Path) {
-            return hasProperty(files, toKey(path));
+            return toKey(path) in files;
         }
 
         function remove(path: Path) {
@@ -55,7 +75,7 @@ namespace ts {
         }
 
         function clear() {
-            files = {};
+            files = createMap<T>();
         }
 
         function toKey(path: Path): string {
@@ -81,7 +101,7 @@ namespace ts {
      * returns a truthy value, then returns that value.
      * If no such value is found, the callback is applied to each element of array and undefined is returned.
      */
-    export function forEach<T, U>(array: T[], callback: (element: T, index: number) => U): U {
+    export function forEach<T, U>(array: T[] | undefined, callback: (element: T, index: number) => U | undefined): U | undefined {
         if (array) {
             for (let i = 0, len = array.length; i < len; i++) {
                 const result = callback(array[i], i);
@@ -93,6 +113,17 @@ namespace ts {
         return undefined;
     }
 
+    /** Like `forEach`, but assumes existence of array and fails if no truthy value is found. */
+    export function find<T, U>(array: T[], callback: (element: T, index: number) => U | undefined): U {
+        for (let i = 0, len = array.length; i < len; i++) {
+            const result = callback(array[i], i);
+            if (result) {
+                return result;
+            }
+        }
+        Debug.fail();
+    }
+
     export function contains<T>(array: T[], value: T): boolean {
         if (array) {
             for (const v of array) {
@@ -136,17 +167,44 @@ namespace ts {
         return count;
     }
 
+    /**
+     * Filters an array by a predicate function. Returns the same array instance if the predicate is
+     * true for all elements, otherwise returns a new array instance containing the filtered subset.
+     */
     export function filter<T>(array: T[], f: (x: T) => boolean): T[] {
-        let result: T[];
         if (array) {
-            result = [];
-            for (const item of array) {
-                if (f(item)) {
-                    result.push(item);
+            const len = array.length;
+            let i = 0;
+            while (i < len && f(array[i])) i++;
+            if (i < len) {
+                const result = array.slice(0, i);
+                i++;
+                while (i < len) {
+                    const item = array[i];
+                    if (f(item)) {
+                        result.push(item);
+                    }
+                    i++;
                 }
+                return result;
             }
         }
-        return result;
+        return array;
+    }
+
+    export function removeWhere<T>(array: T[], f: (x: T) => boolean): boolean {
+        let outIndex = 0;
+        for (const item of array) {
+            if (!f(item)) {
+                array[outIndex] = item;
+                outIndex++;
+            }
+        }
+        if (outIndex !== array.length) {
+            array.length = outIndex;
+            return true;
+        }
+        return false;
     }
 
     export function filterMutate<T>(array: T[], f: (x: T) => boolean): void {
@@ -311,78 +369,142 @@ namespace ts {
 
     const hasOwnProperty = Object.prototype.hasOwnProperty;
 
-    export function hasProperty<T>(map: Map<T>, key: string): boolean {
+    /**
+     * Indicates whether a map-like contains an own property with the specified key.
+     *
+     * NOTE: This is intended for use only with MapLike<T> objects. For Map<T> objects, use
+     *       the 'in' operator.
+     *
+     * @param map A map-like.
+     * @param key A property key.
+     */
+    export function hasProperty<T>(map: MapLike<T>, key: string): boolean {
         return hasOwnProperty.call(map, key);
     }
 
-    export function getKeys<T>(map: Map<T>): string[] {
+    /**
+     * Gets the value of an owned property in a map-like.
+     *
+     * NOTE: This is intended for use only with MapLike<T> objects. For Map<T> objects, use
+     *       an indexer.
+     *
+     * @param map A map-like.
+     * @param key A property key.
+     */
+    export function getProperty<T>(map: MapLike<T>, key: string): T | undefined {
+        return hasOwnProperty.call(map, key) ? map[key] : undefined;
+    }
+
+    /**
+     * Gets the owned, enumerable property keys of a map-like.
+     *
+     * NOTE: This is intended for use with MapLike<T> objects. For Map<T> objects, use
+     *       Object.keys instead as it offers better performance.
+     *
+     * @param map A map-like.
+     */
+    export function getOwnKeys<T>(map: MapLike<T>): string[] {
         const keys: string[] = [];
-        for (const key in map) {
+        for (const key in map) if (hasOwnProperty.call(map, key)) {
             keys.push(key);
         }
         return keys;
     }
 
-    export function getProperty<T>(map: Map<T>, key: string): T {
-        return hasOwnProperty.call(map, key) ? map[key] : undefined;
+    /**
+     * Enumerates the properties of a Map<T>, invoking a callback and returning the first truthy result.
+     *
+     * @param map A map for which properties should be enumerated.
+     * @param callback A callback to invoke for each property.
+     */
+    export function forEachProperty<T, U>(map: Map<T>, callback: (value: T, key: string) => U): U {
+        let result: U;
+        for (const key in map) {
+            if (result = callback(map[key], key)) break;
+        }
+        return result;
     }
 
-    export function isEmpty<T>(map: Map<T>) {
-        for (const id in map) {
-            if (hasProperty(map, id)) {
-                return false;
-            }
+    /**
+     * Returns true if a Map<T> has some matching property.
+     *
+     * @param map A map whose properties should be tested.
+     * @param predicate An optional callback used to test each property.
+     */
+    export function someProperties<T>(map: Map<T>, predicate?: (value: T, key: string) => boolean) {
+        for (const key in map) {
+            if (!predicate || predicate(map[key], key)) return true;
+        }
+        return false;
+    }
+
+    /**
+     * Performs a shallow copy of the properties from a source Map<T> to a target MapLike<T>
+     *
+     * @param source A map from which properties should be copied.
+     * @param target A map to which properties should be copied.
+     */
+    export function copyProperties<T>(source: Map<T>, target: MapLike<T>): void {
+        for (const key in source) {
+            target[key] = source[key];
+        }
+    }
+
+    /**
+     * Reduce the properties of a map.
+     *
+     * NOTE: This is intended for use with Map<T> objects. For MapLike<T> objects, use
+     *       reduceOwnProperties instead as it offers better runtime safety.
+     *
+     * @param map The map to reduce
+     * @param callback An aggregation function that is called for each entry in the map
+     * @param initial The initial value for the reduction.
+     */
+    export function reduceProperties<T, U>(map: Map<T>, callback: (aggregate: U, value: T, key: string) => U, initial: U): U {
+        let result = initial;
+        for (const key in map) {
+            result = callback(result, map[key], String(key));
+        }
+        return result;
+    }
+
+    /**
+     * Reduce the properties defined on a map-like (but not from its prototype chain).
+     *
+     * NOTE: This is intended for use with MapLike<T> objects. For Map<T> objects, use
+     *       reduceProperties instead as it offers better performance.
+     *
+     * @param map The map-like to reduce
+     * @param callback An aggregation function that is called for each entry in the map
+     * @param initial The initial value for the reduction.
+     */
+    export function reduceOwnProperties<T, U>(map: MapLike<T>, callback: (aggregate: U, value: T, key: string) => U, initial: U): U {
+        let result = initial;
+        for (const key in map) if (hasOwnProperty.call(map, key)) {
+            result = callback(result, map[key], String(key));
+        }
+        return result;
+    }
+
+    /**
+     * Performs a shallow equality comparison of the contents of two map-likes.
+     *
+     * @param left A map-like whose properties should be compared.
+     * @param right A map-like whose properties should be compared.
+     */
+    export function equalOwnProperties<T>(left: MapLike<T>, right: MapLike<T>, equalityComparer?: (left: T, right: T) => boolean) {
+        if (left === right) return true;
+        if (!left || !right) return false;
+        for (const key in left) if (hasOwnProperty.call(left, key)) {
+            if (!hasOwnProperty.call(right, key) === undefined) return false;
+            if (equalityComparer ? !equalityComparer(left[key], right[key]) : left[key] !== right[key]) return false;
+        }
+        for (const key in right) if (hasOwnProperty.call(right, key)) {
+            if (!hasOwnProperty.call(left, key)) return false;
         }
         return true;
     }
 
-    export function clone<T>(object: T): T {
-        const result: any = {};
-        for (const id in object) {
-            result[id] = (<any>object)[id];
-        }
-        return <T>result;
-    }
-
-    export function extend<T1 extends Map<{}>, T2 extends Map<{}>>(first: T1 , second: T2): T1 & T2 {
-        const result: T1 & T2 = <any>{};
-        for (const id in first) {
-            (result as any)[id] = first[id];
-        }
-        for (const id in second) {
-            if (!hasProperty(result, id)) {
-                (result as any)[id] = second[id];
-            }
-        }
-        return result;
-    }
-
-    export function forEachValue<T, U>(map: Map<T>, callback: (value: T) => U): U {
-        let result: U;
-        for (const id in map) {
-            if (result = callback(map[id])) break;
-        }
-        return result;
-    }
-
-    export function forEachKey<T, U>(map: Map<T>, callback: (key: string) => U): U {
-        let result: U;
-        for (const id in map) {
-            if (result = callback(id)) break;
-        }
-        return result;
-    }
-
-    export function lookUp<T>(map: Map<T>, key: string): T {
-        return hasProperty(map, key) ? map[key] : undefined;
-    }
-
-    export function copyMap<T>(source: Map<T>, target: Map<T>): void {
-        for (const p in source) {
-            target[p] = source[p];
-        }
-    }
-
     /**
      * Creates a map from the elements of an array.
      *
@@ -393,33 +515,40 @@ namespace ts {
      * the same key with the given 'makeKey' function, then the element with the higher
      * index in the array will be the one associated with the produced key.
      */
-    export function arrayToMap<T>(array: T[], makeKey: (value: T) => string): Map<T> {
-        const result: Map<T> = {};
-
-        forEach(array, value => {
-            result[makeKey(value)] = value;
-        });
-
+    export function arrayToMap<T>(array: T[], makeKey: (value: T) => string): Map<T>;
+    export function arrayToMap<T, U>(array: T[], makeKey: (value: T) => string, makeValue: (value: T) => U): Map<U>;
+    export function arrayToMap<T, U>(array: T[], makeKey: (value: T) => string, makeValue?: (value: T) => U): Map<T | U> {
+        const result = createMap<T | U>();
+        for (const value of array) {
+            result[makeKey(value)] = makeValue ? makeValue(value) : value;
+        }
         return result;
     }
 
-    /**
-     * Reduce the properties of a map.
-     *
-     * @param map The map to reduce
-     * @param callback An aggregation function that is called for each entry in the map
-     * @param initial The initial value for the reduction.
-     */
-    export function reduceProperties<T, U>(map: Map<T>, callback: (aggregate: U, value: T, key: string) => U, initial: U): U {
-        let result = initial;
-        if (map) {
-            for (const key in map) {
-                if (hasProperty(map, key)) {
-                    result = callback(result, map[key], String(key));
-                }
+    export function cloneMap<T>(map: Map<T>) {
+        const clone = createMap<T>();
+        copyProperties(map, clone);
+        return clone;
+    }
+
+    export function clone<T>(object: T): T {
+        const result: any = {};
+        for (const id in object) {
+            if (hasOwnProperty.call(object, id)) {
+                result[id] = (<any>object)[id];
             }
         }
+        return result;
+    }
 
+    export function extend<T1, T2>(first: T1 , second: T2): T1 & T2 {
+        const result: T1 & T2 = <any>{};
+        for (const id in second) if (hasOwnProperty.call(second, id)) {
+            (result as any)[id] = (second as any)[id];
+        }
+        for (const id in first) if (hasOwnProperty.call(first, id)) {
+            (result as any)[id] = (first as any)[id];
+        }
         return result;
     }
 
@@ -450,9 +579,7 @@ namespace ts {
     export let localizedDiagnosticMessages: Map<string> = undefined;
 
     export function getLocaleSpecificMessage(message: DiagnosticMessage) {
-        return localizedDiagnosticMessages && localizedDiagnosticMessages[message.key]
-            ? localizedDiagnosticMessages[message.key]
-            : message.message;
+        return localizedDiagnosticMessages && localizedDiagnosticMessages[message.key] || message.message;
     }
 
     export function createFileDiagnostic(file: SourceFile, start: number, length: number, message: DiagnosticMessage, ...args: any[]): Diagnostic;
@@ -941,7 +1068,7 @@ namespace ts {
      *  [^./]                   # matches everything up to the first . character (excluding directory seperators)
      *  (\\.(?!min\\.js$))?     # matches . characters but not if they are part of the .min.js file extension
      */
-    const singleAsteriskRegexFragmentFiles = "([^./]|(\\.(?!min\\.js$))?)*";
+    const singleAsteriskRegexFragmentFiles = "([^./]|(\\.(?!min\\.js$))?)*";
     const singleAsteriskRegexFragmentOther = "[^/]*";
 
     export function getRegularExpressionForWildcard(specs: string[], basePath: string, usage: "files" | "directories" | "exclude") {
diff --git a/src/compiler/declarationEmitter.ts b/src/compiler/declarationEmitter.ts
index d93a8a0aed0..3642ee7f10a 100644
--- a/src/compiler/declarationEmitter.ts
+++ b/src/compiler/declarationEmitter.ts
@@ -157,9 +157,7 @@ namespace ts {
 
         if (usedTypeDirectiveReferences) {
             for (const directive in usedTypeDirectiveReferences) {
-                if (hasProperty(usedTypeDirectiveReferences, directive)) {
-                    referencesOutput += `/// <reference types="${directive}" />${newLine}`;
-                }
+                referencesOutput += `/// <reference types="${directive}" />${newLine}`;
             }
         }
 
@@ -269,10 +267,10 @@ namespace ts {
             }
 
             if (!usedTypeDirectiveReferences) {
-                usedTypeDirectiveReferences = {};
+                usedTypeDirectiveReferences = createMap<string>();
             }
             for (const directive of typeReferenceDirectives) {
-                if (!hasProperty(usedTypeDirectiveReferences, directive)) {
+                if (!(directive in usedTypeDirectiveReferences)) {
                     usedTypeDirectiveReferences[directive] = directive;
                 }
             }
@@ -441,7 +439,7 @@ namespace ts {
                 }
             }
 
-            function emitEntityName(entityName: EntityName | PropertyAccessExpression) {
+            function emitEntityName(entityName: EntityNameOrEntityNameExpression) {
                 const visibilityResult = resolver.isEntityNameVisible(entityName,
                     // Aliases can be written asynchronously so use correct enclosing declaration
                     entityName.parent.kind === SyntaxKind.ImportEqualsDeclaration ? entityName.parent : enclosingDeclaration);
@@ -452,9 +450,9 @@ namespace ts {
             }
 
             function emitExpressionWithTypeArguments(node: ExpressionWithTypeArguments) {
-                if (isSupportedExpressionWithTypeArguments(node)) {
+                if (isEntityNameExpression(node.expression)) {
                     Debug.assert(node.expression.kind === SyntaxKind.Identifier || node.expression.kind === SyntaxKind.PropertyAccessExpression);
-                    emitEntityName(<Identifier | PropertyAccessExpression>node.expression);
+                    emitEntityName(node.expression);
                     if (node.typeArguments) {
                         write("<");
                         emitCommaList(node.typeArguments, emitType);
@@ -537,14 +535,14 @@ namespace ts {
         // do not need to keep track of created temp names.
         function getExportDefaultTempVariableName(): string {
             const baseName = "_default";
-            if (!hasProperty(currentIdentifiers, baseName)) {
+            if (!(baseName in currentIdentifiers)) {
                 return baseName;
             }
             let count = 0;
             while (true) {
                 count++;
                 const name = baseName + "_" + count;
-                if (!hasProperty(currentIdentifiers, name)) {
+                if (!(name in currentIdentifiers)) {
                     return name;
                 }
             }
@@ -1019,7 +1017,7 @@ namespace ts {
             }
 
             function emitTypeOfTypeReference(node: ExpressionWithTypeArguments) {
-                if (isSupportedExpressionWithTypeArguments(node)) {
+                if (isEntityNameExpression(node.expression)) {
                     emitTypeWithNewGetSymbolAccessibilityDiagnostic(node, getHeritageClauseVisibilityError);
                 }
                 else if (!isImplementsList && node.expression.kind === SyntaxKind.NullKeyword) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 8126d5c605e..c6a3698ea16 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2231,7 +2231,7 @@
         "category": "Error",
         "code": 4082
     },
-    "Conflicting library definitions for '{0}' found at '{1}' and '{2}'. Copy the correct file to the 'typings' folder to resolve this conflict.": {
+    "Conflicting definitions for '{0}' found at '{1}' and '{2}'. Consider installing a specific version of this library to resolve the conflict.": {
         "category": "Message",
         "code": 4090
     },
@@ -2828,6 +2828,10 @@
         "category": "Message",
         "code": 6137
     },
+    "Property '{0}' is declared but never used.": {
+        "category": "Error",
+        "code": 6138
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 5fe5027978c..869f2decce7 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -24,7 +24,7 @@ namespace ts {
         Return      = 1 << 3
     }
 
-    const entities: Map<number> = {
+    const entities = createMap({
         "quot": 0x0022,
         "amp": 0x0026,
         "apos": 0x0027,
@@ -278,7 +278,7 @@ namespace ts {
         "clubs": 0x2663,
         "hearts": 0x2665,
         "diams": 0x2666
-    };
+    });
 
     // Flags enum to track count of temp variables and a few dedicated names
     const enum TempFlags {
@@ -407,7 +407,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 
         function isUniqueLocalName(name: string, container: Node): boolean {
             for (let node = container; isNodeDescendentOf(node, container); node = node.nextContainer) {
-                if (node.locals && hasProperty(node.locals, name)) {
+                if (node.locals && name in node.locals) {
                     // We conservatively include alias symbols to cover cases where they're emitted as locals
                     if (node.locals[name].flags & (SymbolFlags.Value | SymbolFlags.ExportValue | SymbolFlags.Alias)) {
                         return false;
@@ -489,13 +489,13 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
         function setLabeledJump(state: ConvertedLoopState, isBreak: boolean, labelText: string, labelMarker: string): void {
             if (isBreak) {
                 if (!state.labeledNonLocalBreaks) {
-                    state.labeledNonLocalBreaks = {};
+                    state.labeledNonLocalBreaks = createMap<string>();
                 }
                 state.labeledNonLocalBreaks[labelText] = labelMarker;
             }
             else {
                 if (!state.labeledNonLocalContinues) {
-                    state.labeledNonLocalContinues = {};
+                    state.labeledNonLocalContinues = createMap<string>();
                 }
                 state.labeledNonLocalContinues[labelText] = labelMarker;
             }
@@ -577,27 +577,27 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 
             const setSourceMapWriterEmit = compilerOptions.sourceMap || compilerOptions.inlineSourceMap ? changeSourceMapEmit : function (writer: SourceMapWriter) { };
 
-            const moduleEmitDelegates: Map<(node: SourceFile, emitRelativePathAsModuleName?: boolean) => void> = {
+            const moduleEmitDelegates = createMap<(node: SourceFile, emitRelativePathAsModuleName?: boolean) => void>({
                 [ModuleKind.ES6]: emitES6Module,
                 [ModuleKind.AMD]: emitAMDModule,
                 [ModuleKind.System]: emitSystemModule,
                 [ModuleKind.UMD]: emitUMDModule,
                 [ModuleKind.CommonJS]: emitCommonJSModule,
-            };
+            });
 
-            const bundleEmitDelegates: Map<(node: SourceFile, emitRelativePathAsModuleName?: boolean) => void> = {
+            const bundleEmitDelegates = createMap<(node: SourceFile, emitRelativePathAsModuleName?: boolean) => void>({
                 [ModuleKind.ES6]() {},
                 [ModuleKind.AMD]: emitAMDModule,
                 [ModuleKind.System]: emitSystemModule,
                 [ModuleKind.UMD]() {},
                 [ModuleKind.CommonJS]() {},
-            };
+            });
 
             return doEmit;
 
             function doEmit(jsFilePath: string, sourceMapFilePath: string, sourceFiles: SourceFile[], isBundledEmit: boolean) {
                 sourceMap.initialize(jsFilePath, sourceMapFilePath, sourceFiles, isBundledEmit);
-                generatedNameSet = {};
+                generatedNameSet = createMap<string>();
                 nodeToGeneratedName = [];
                 decoratedClassAliases = [];
                 isOwnFileEmit = !isBundledEmit;
@@ -669,8 +669,8 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 
             function isUniqueName(name: string): boolean {
                 return !resolver.hasGlobalName(name) &&
-                    !hasProperty(currentFileIdentifiers, name) &&
-                    !hasProperty(generatedNameSet, name);
+                    !(name in currentFileIdentifiers) &&
+                    !(name in generatedNameSet);
             }
 
             // Return the next available name in the pattern _a ... _z, _0, _1, ...
@@ -2646,7 +2646,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
                     return false;
                 }
 
-                return !exportEquals && exportSpecifiers && hasProperty(exportSpecifiers, (<Identifier>node).text);
+                return !exportEquals && exportSpecifiers && (<Identifier>node).text in exportSpecifiers;
             }
 
             function emitPrefixUnaryExpression(node: PrefixUnaryExpression) {
@@ -3257,13 +3257,13 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
                            // Don't initialize seen unless we have at least one element.
                            // Emit a comma to separate for all but the first element.
                            if (!seen) {
-                               seen = {};
+                               seen = createMap<string>();
                            }
                            else {
                                write(", ");
                            }
 
-                           if (!hasProperty(seen, id.text)) {
+                           if (!(id.text in seen)) {
                                emit(id);
                                seen[id.text] = id.text;
                            }
@@ -3856,7 +3856,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
 
                 if (convertedLoopState) {
                     if (!convertedLoopState.labels) {
-                        convertedLoopState.labels = {};
+                        convertedLoopState.labels = createMap<string>();
                     }
                     convertedLoopState.labels[node.label.text] = node.label.text;
                 }
@@ -3970,7 +3970,7 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
                     return;
                 }
 
-                if (!exportEquals && exportSpecifiers && hasProperty(exportSpecifiers, name.text)) {
+                if (!exportEquals && exportSpecifiers && name.text in exportSpecifiers) {
                     for (const specifier of exportSpecifiers[name.text]) {
                         writeLine();
                         emitStart(specifier.name);
@@ -5311,18 +5311,7 @@ const _super = (function (geti, seti) {
                         emitSignatureParameters(ctor);
                     }
                     else {
-                        // Based on EcmaScript6 section 14.5.14: Runtime Semantics: ClassDefinitionEvaluation.
-                        // If constructor is empty, then,
-                        //      If ClassHeritageopt is present, then
-                        //          Let constructor be the result of parsing the String "constructor(... args){ super (...args);}" using the syntactic grammar with the goal symbol MethodDefinition.
-                        //      Else,
-                        //          Let constructor be the result of parsing the String "constructor( ){ }" using the syntactic grammar with the goal symbol MethodDefinition
-                        if (baseTypeElement) {
-                            write("(...args)");
-                        }
-                        else {
-                            write("()");
-                        }
+                        write("()");
                     }
                 }
 
@@ -5360,7 +5349,7 @@ const _super = (function (geti, seti) {
                             write("_super.apply(this, arguments);");
                         }
                         else {
-                            write("super(...args);");
+                            write("super(...arguments);");
                         }
                         emitEnd(baseTypeElement);
                     }
@@ -6461,7 +6450,7 @@ const _super = (function (geti, seti) {
              * Here we check if alternative name was provided for a given moduleName and return it if possible.
              */
             function tryRenameExternalModule(moduleName: LiteralExpression): string {
-                if (renamedDependencies && hasProperty(renamedDependencies, moduleName.text)) {
+                if (renamedDependencies && moduleName.text in renamedDependencies) {
                     return `"${renamedDependencies[moduleName.text]}"`;
                 }
                 return undefined;
@@ -6803,7 +6792,7 @@ const _super = (function (geti, seti) {
 
             function collectExternalModuleInfo(sourceFile: SourceFile) {
                 externalImports = [];
-                exportSpecifiers = {};
+                exportSpecifiers = createMap<ExportSpecifier[]>();
                 exportEquals = undefined;
                 hasExportStarsToExportValues = false;
                 for (const node of sourceFile.statements) {
@@ -6941,7 +6930,7 @@ const _super = (function (geti, seti) {
                 }
 
                 // local names set should only be added if we have anything exported
-                if (!exportedDeclarations && isEmpty(exportSpecifiers)) {
+                if (!exportedDeclarations && !someProperties(exportSpecifiers)) {
                     // no exported declarations (export var ...) or export specifiers (export {x})
                     // check if we have any non star export declarations.
                     let hasExportDeclarationWithExportClause = false;
@@ -7081,7 +7070,7 @@ const _super = (function (geti, seti) {
                 if (hoistedVars) {
                     writeLine();
                     write("var ");
-                    const seen: Map<string> = {};
+                    const seen = createMap<string>();
                     for (let i = 0; i < hoistedVars.length; i++) {
                         const local = hoistedVars[i];
                         const name = local.kind === SyntaxKind.Identifier
@@ -7091,7 +7080,7 @@ const _super = (function (geti, seti) {
                         if (name) {
                             // do not emit duplicate entries (in case of declaration merging) in the list of hoisted variables
                             const text = unescapeIdentifier(name.text);
-                            if (hasProperty(seen, text)) {
+                            if (text in seen) {
                                 continue;
                             }
                             else {
@@ -7447,7 +7436,7 @@ const _super = (function (geti, seti) {
                 writeModuleName(node, emitRelativePathAsModuleName);
                 write("[");
 
-                const groupIndices: Map<number> = {};
+                const groupIndices = createMap<number>();
                 const dependencyGroups: DependencyGroup[] = [];
 
                 for (let i = 0; i < externalImports.length; i++) {
@@ -7460,7 +7449,7 @@ const _super = (function (geti, seti) {
                     // for deduplication purposes in key remove leading and trailing quotes so 'a' and "a" will be considered the same
                     const key = text.substr(1, text.length - 2);
 
-                    if (hasProperty(groupIndices, key)) {
+                    if (key in groupIndices) {
                         // deduplicate/group entries in dependency list by the dependency name
                         const groupIndex = groupIndices[key];
                         dependencyGroups[groupIndex].push(externalImports[i]);
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 014d0926213..aca7d745730 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -421,10 +421,10 @@ namespace ts {
     }
 
     export function createSourceFile(fileName: string, sourceText: string, languageVersion: ScriptTarget, setParentNodes = false, scriptKind?: ScriptKind): SourceFile {
-        const start = performance.mark();
+        performance.mark("beforeParse");
         const result = Parser.parseSourceFile(fileName, sourceText, languageVersion, /*syntaxCursor*/ undefined, setParentNodes, scriptKind);
-
-        performance.measure("Parse", start);
+        performance.mark("afterParse");
+        performance.measure("Parse", "beforeParse", "afterParse");
         return result;
     }
 
@@ -595,7 +595,7 @@ namespace ts {
 
             parseDiagnostics = [];
             parsingContext = 0;
-            identifiers = {};
+            identifiers = createMap<string>();
             identifierCount = 0;
             nodeCount = 0;
 
@@ -1084,7 +1084,7 @@ namespace ts {
 
         function internIdentifier(text: string): string {
             text = escapeIdentifier(text);
-            return hasProperty(identifiers, text) ? identifiers[text] : (identifiers[text] = text);
+            return identifiers[text] || (identifiers[text] = text);
         }
 
         // An identifier that starts with two underscores has an extra underscore character prepended to it to avoid issues
diff --git a/src/compiler/performance.ts b/src/compiler/performance.ts
index 89db876ae5e..e8f064e81cd 100644
--- a/src/compiler/performance.ts
+++ b/src/compiler/performance.ts
@@ -6,71 +6,57 @@ namespace ts {
 }
 
 /*@internal*/
+/** Performance measurements for the compiler. */
 namespace ts.performance {
-    /** Performance measurements for the compiler. */
     declare const onProfilerEvent: { (markName: string): void; profiler: boolean; };
-    let profilerEvent: (markName: string) => void;
-    let counters: Map<number>;
+
+    const profilerEvent = typeof onProfilerEvent === "function" && onProfilerEvent.profiler === true
+            ? onProfilerEvent
+            : (markName: string) => { };
+
+    let enabled = false;
+    let profilerStart = 0;
+    let counts: Map<number>;
+    let marks: Map<number>;
     let measures: Map<number>;
 
     /**
-     * Emit a performance event if ts-profiler is connected. This is primarily used
-     * to generate heap snapshots.
+     * Marks a performance event.
      *
-     * @param eventName A name for the event.
+     * @param markName The name of the mark.
      */
-    export function emit(eventName: string) {
-        if (profilerEvent) {
-            profilerEvent(eventName);
+    export function mark(markName: string) {
+        if (enabled) {
+            marks[markName] = timestamp();
+            counts[markName] = (counts[markName] || 0) + 1;
+            profilerEvent(markName);
         }
     }
 
-    /**
-     * Increments a counter with the specified name.
-     *
-     * @param counterName The name of the counter.
-     */
-    export function increment(counterName: string) {
-        if (counters) {
-            counters[counterName] = (getProperty(counters, counterName) || 0) + 1;
-        }
-    }
-
-    /**
-     * Gets the value of the counter with the specified name.
-     *
-     * @param counterName The name of the counter.
-     */
-    export function getCount(counterName: string) {
-        return counters && getProperty(counters, counterName) || 0;
-    }
-
-    /**
-     * Marks the start of a performance measurement.
-     */
-    export function mark() {
-        return measures ? timestamp() : 0;
-    }
-
     /**
      * Adds a performance measurement with the specified name.
      *
      * @param measureName The name of the performance measurement.
-     * @param marker The timestamp of the starting mark.
+     * @param startMarkName The name of the starting mark. If not supplied, the point at which the
+     *      profiler was enabled is used.
+     * @param endMarkName The name of the ending mark. If not supplied, the current timestamp is
+     *      used.
      */
-    export function measure(measureName: string, marker: number) {
-        if (measures) {
-            measures[measureName] = (getProperty(measures, measureName) || 0) + (timestamp() - marker);
+    export function measure(measureName: string, startMarkName?: string, endMarkName?: string) {
+        if (enabled) {
+            const end = endMarkName && marks[endMarkName] || timestamp();
+            const start = startMarkName && marks[startMarkName] || profilerStart;
+            measures[measureName] = (measures[measureName] || 0) + (end - start);
         }
     }
 
     /**
-     * Iterate over each measure, performing some action
-     * 
-     * @param cb The action to perform for each measure
+     * Gets the number of times a marker was encountered.
+     *
+     * @param markName The name of the mark.
      */
-    export function forEachMeasure(cb: (measureName: string, duration: number) => void) {
-        return forEachKey(measures, key => cb(key, measures[key]));
+    export function getCount(markName: string) {
+        return counts && counts[markName] || 0;
     }
 
     /**
@@ -79,31 +65,31 @@ namespace ts.performance {
      * @param measureName The name of the measure whose durations should be accumulated.
      */
     export function getDuration(measureName: string) {
-        return measures && getProperty(measures, measureName) || 0;
+        return measures && measures[measureName] || 0;
+    }
+
+    /**
+     * Iterate over each measure, performing some action
+     *
+     * @param cb The action to perform for each measure
+     */
+    export function forEachMeasure(cb: (measureName: string, duration: number) => void) {
+        for (const key in measures) {
+            cb(key, measures[key]);
+        }
     }
 
     /** Enables (and resets) performance measurements for the compiler. */
     export function enable() {
-        counters = { };
-        measures = {
-            "I/O Read": 0,
-            "I/O Write": 0,
-            "Program": 0,
-            "Parse": 0,
-            "Bind": 0,
-            "Check": 0,
-            "Emit": 0,
-        };
-
-        profilerEvent = typeof onProfilerEvent === "function" && onProfilerEvent.profiler === true
-            ? onProfilerEvent
-            : undefined;
+        counts = createMap<number>();
+        marks = createMap<number>();
+        measures = createMap<number>();
+        enabled = true;
+        profilerStart = timestamp();
     }
 
-    /** Disables (and clears) performance measurements for the compiler. */
+    /** Disables performance measurements for the compiler. */
     export function disable() {
-        counters = undefined;
-        measures = undefined;
-        profilerEvent = undefined;
+        enabled = false;
     }
 }
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 4b9948c3ba0..004f4102667 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -119,49 +119,31 @@ namespace ts {
     }
 
     function tryReadTypesSection(packageJsonPath: string, baseDirectory: string, state: ModuleResolutionState): string {
-        let jsonContent: { typings?: string, types?: string, main?: string };
-        try {
-            const jsonText = state.host.readFile(packageJsonPath);
-            jsonContent = jsonText ? <{ typings?: string, types?: string, main?: string }>JSON.parse(jsonText) : {};
-        }
-        catch (e) {
-            // gracefully handle if readFile fails or returns not JSON
-            jsonContent = {};
+        const jsonContent = readJson(packageJsonPath, state.host);
+
+        function tryReadFromField(fieldName: string) {
+            if (hasProperty(jsonContent, fieldName)) {
+                const typesFile = (<any>jsonContent)[fieldName];
+                if (typeof typesFile === "string") {
+                    const typesFilePath = normalizePath(combinePaths(baseDirectory, typesFile));
+                    if (state.traceEnabled) {
+                        trace(state.host, Diagnostics.package_json_has_0_field_1_that_references_2, fieldName, typesFile, typesFilePath);
+                    }
+                    return typesFilePath;
+                }
+                else {
+                    if (state.traceEnabled) {
+                        trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_string_got_1, fieldName, typeof typesFile);
+                    }
+                }
+            }
         }
 
-        let typesFile: string;
-        let fieldName: string;
-        // first try to read content of 'typings' section (backward compatibility)
-        if (jsonContent.typings) {
-            if (typeof jsonContent.typings === "string") {
-                fieldName = "typings";
-                typesFile = jsonContent.typings;
-            }
-            else {
-                if (state.traceEnabled) {
-                    trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_string_got_1, "typings", typeof jsonContent.typings);
-                }
-            }
-        }
-        // then read 'types'
-        if (!typesFile && jsonContent.types) {
-            if (typeof jsonContent.types === "string") {
-                fieldName = "types";
-                typesFile = jsonContent.types;
-            }
-            else {
-                if (state.traceEnabled) {
-                    trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_string_got_1, "types", typeof jsonContent.types);
-                }
-            }
-        }
-        if (typesFile) {
-            const typesFilePath = normalizePath(combinePaths(baseDirectory, typesFile));
-            if (state.traceEnabled) {
-                trace(state.host, Diagnostics.package_json_has_0_field_1_that_references_2, fieldName, typesFile, typesFilePath);
-            }
+        const typesFilePath = tryReadFromField("typings") || tryReadFromField("types");
+        if (typesFilePath) {
             return typesFilePath;
         }
+
         // Use the main module for inferring types if no types package specified and the allowJs is set
         if (state.compilerOptions.allowJs && jsonContent.main && typeof jsonContent.main === "string") {
             if (state.traceEnabled) {
@@ -173,6 +155,17 @@ namespace ts {
         return undefined;
     }
 
+    function readJson(path: string, host: ModuleResolutionHost): { typings?: string, types?: string, main?: string } {
+        try {
+            const jsonText = host.readFile(path);
+            return jsonText ? JSON.parse(jsonText) : {};
+        }
+        catch (e) {
+            // gracefully handle if readFile fails or returns not JSON
+            return {};
+        }
+    }
+
     const typeReferenceExtensions = [".d.ts"];
 
     function getEffectiveTypeRoots(options: CompilerOptions, host: ModuleResolutionHost) {
@@ -508,7 +501,7 @@ namespace ts {
             if (state.traceEnabled) {
                 trace(state.host, Diagnostics.paths_option_is_specified_looking_for_a_pattern_to_match_module_name_0, moduleName);
             }
-            matchedPattern = matchPatternOrExact(getKeys(state.compilerOptions.paths), moduleName);
+            matchedPattern = matchPatternOrExact(getOwnKeys(state.compilerOptions.paths), moduleName);
         }
 
         if (matchedPattern) {
@@ -717,7 +710,7 @@ namespace ts {
     }
 
     function loadNodeModuleFromDirectory(extensions: string[], candidate: string, failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState): string {
-        const packageJsonPath = combinePaths(candidate, "package.json");
+        const packageJsonPath = pathToPackageJson(candidate);
         const directoryExists = !onlyRecordFailures && directoryProbablyExists(candidate, state.host);
         if (directoryExists && state.host.fileExists(packageJsonPath)) {
             if (state.traceEnabled) {
@@ -747,6 +740,10 @@ namespace ts {
         return loadModuleFromFile(combinePaths(candidate, "index"), extensions, failedLookupLocation, !directoryExists, state);
     }
 
+    function pathToPackageJson(directory: string): string {
+        return combinePaths(directory, "package.json");
+    }
+
     function loadModuleFromNodeModulesFolder(moduleName: string, directory: string, failedLookupLocations: string[], state: ModuleResolutionState): string {
         const nodeModulesFolder = combinePaths(directory, "node_modules");
         const nodeModulesFolderExists = directoryProbablyExists(nodeModulesFolder, state.host);
@@ -846,7 +843,7 @@ namespace ts {
     }
 
     export function createCompilerHost(options: CompilerOptions, setParentNodes?: boolean): CompilerHost {
-        const existingDirectories: Map<boolean> = {};
+        const existingDirectories = createMap<boolean>();
 
         function getCanonicalFileName(fileName: string): string {
             // if underlying system can distinguish between two files whose names differs only in cases then file name already in canonical form.
@@ -860,9 +857,10 @@ namespace ts {
         function getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void): SourceFile {
             let text: string;
             try {
-                const start = performance.mark();
+                performance.mark("beforeIORead");
                 text = sys.readFile(fileName, options.charset);
-                performance.measure("I/O Read", start);
+                performance.mark("afterIORead");
+                performance.measure("I/O Read", "beforeIORead", "afterIORead");
             }
             catch (e) {
                 if (onError) {
@@ -877,7 +875,7 @@ namespace ts {
         }
 
         function directoryExists(directoryPath: string): boolean {
-            if (hasProperty(existingDirectories, directoryPath)) {
+            if (directoryPath in existingDirectories) {
                 return true;
             }
             if (sys.directoryExists(directoryPath)) {
@@ -899,13 +897,13 @@ namespace ts {
 
         function writeFileIfUpdated(fileName: string, data: string, writeByteOrderMark: boolean): void {
             if (!outputFingerprints) {
-                outputFingerprints = {};
+                outputFingerprints = createMap<OutputFingerprint>();
             }
 
             const hash = sys.createHash(data);
             const mtimeBefore = sys.getModifiedTime(fileName);
 
-            if (mtimeBefore && hasProperty(outputFingerprints, fileName)) {
+            if (mtimeBefore && fileName in outputFingerprints) {
                 const fingerprint = outputFingerprints[fileName];
 
                 // If output has not been changed, and the file has no external modification
@@ -929,7 +927,7 @@ namespace ts {
 
         function writeFile(fileName: string, data: string, writeByteOrderMark: boolean, onError?: (message: string) => void) {
             try {
-                const start = performance.mark();
+                performance.mark("beforeIOWrite");
                 ensureDirectoriesExist(getDirectoryPath(normalizePath(fileName)));
 
                 if (isWatchSet(options) && sys.createHash && sys.getModifiedTime) {
@@ -939,7 +937,8 @@ namespace ts {
                     sys.writeFile(fileName, data, writeByteOrderMark);
                 }
 
-                performance.measure("I/O Write", start);
+                performance.mark("afterIOWrite");
+                performance.measure("I/O Write", "beforeIOWrite", "afterIOWrite");
             }
             catch (e) {
                 if (onError) {
@@ -1040,47 +1039,47 @@ namespace ts {
             return [];
         }
         const resolutions: T[] = [];
-        const cache: Map<T> = {};
+        const cache = createMap<T>();
         for (const name of names) {
-            let result: T;
-            if (hasProperty(cache, name)) {
-                result = cache[name];
-            }
-            else {
-                result = loader(name, containingFile);
-                cache[name] = result;
-            }
+            const result = name in cache
+                ? cache[name]
+                : cache[name] = loader(name, containingFile);
             resolutions.push(result);
         }
         return resolutions;
     }
 
-    function getInferredTypesRoot(options: CompilerOptions, rootFiles: string[], host: CompilerHost) {
-        return computeCommonSourceDirectoryOfFilenames(rootFiles, host.getCurrentDirectory(), f => host.getCanonicalFileName(f));
-    }
-
     /**
-      * Given a set of options and a set of root files, returns the set of type directive names
+      * Given a set of options, returns the set of type directive names
       *   that should be included for this program automatically.
       * This list could either come from the config file,
       *   or from enumerating the types root + initial secondary types lookup location.
       * More type directives might appear in the program later as a result of loading actual source files;
       *   this list is only the set of defaults that are implicitly included.
       */
-    export function getAutomaticTypeDirectiveNames(options: CompilerOptions, rootFiles: string[], host: CompilerHost): string[] {
+    export function getAutomaticTypeDirectiveNames(options: CompilerOptions, host: ModuleResolutionHost): string[] {
         // Use explicit type list from tsconfig.json
         if (options.types) {
             return options.types;
         }
 
         // Walk the primary type lookup locations
-        let result: string[] = [];
+        const result: string[] = [];
         if (host.directoryExists && host.getDirectories) {
             const typeRoots = getEffectiveTypeRoots(options, host);
             if (typeRoots) {
                 for (const root of typeRoots) {
                     if (host.directoryExists(root)) {
-                        result = result.concat(host.getDirectories(root));
+                        for (const typeDirectivePath of host.getDirectories(root)) {
+                            const normalized = normalizePath(typeDirectivePath);
+                            const packageJsonPath = pathToPackageJson(combinePaths(root, normalized));
+                            // tslint:disable-next-line:no-null-keyword
+                            const isNotNeededPackage = host.fileExists(packageJsonPath) && readJson(packageJsonPath, host).typings === null;
+                            if (!isNotNeededPackage) {
+                                // Return just the type directive names
+                                result.push(getBaseFileName(normalized));
+                            }
+                        }
                     }
                 }
             }
@@ -1096,7 +1095,7 @@ namespace ts {
         let noDiagnosticsTypeChecker: TypeChecker;
         let classifiableNames: Map<string>;
 
-        let resolvedTypeReferenceDirectives: Map<ResolvedTypeReferenceDirective> = {};
+        let resolvedTypeReferenceDirectives = createMap<ResolvedTypeReferenceDirective>();
         let fileProcessingDiagnostics = createDiagnosticCollection();
 
         // The below settings are to track if a .js file should be add to the program if loaded via searching under node_modules.
@@ -1111,12 +1110,12 @@ namespace ts {
 
         // If a module has some of its imports skipped due to being at the depth limit under node_modules, then track
         // this, as it may be imported at a shallower depth later, and then it will need its skipped imports processed.
-        const modulesWithElidedImports: Map<boolean> = {};
+        const modulesWithElidedImports = createMap<boolean>();
 
         // Track source files that are source files found by searching under node_modules, as these shouldn't be compiled.
-        const sourceFilesFoundSearchingNodeModules: Map<boolean> = {};
+        const sourceFilesFoundSearchingNodeModules = createMap<boolean>();
 
-        const start = performance.mark();
+        performance.mark("beforeProgram");
 
         host = host || createCompilerHost(options);
 
@@ -1155,11 +1154,11 @@ namespace ts {
             forEach(rootNames, name => processRootFile(name, /*isDefaultLib*/ false));
 
             // load type declarations specified via 'types' argument or implicitly from types/ and node_modules/@types folders
-            const typeReferences: string[] = getAutomaticTypeDirectiveNames(options, rootNames, host);
+            const typeReferences: string[] = getAutomaticTypeDirectiveNames(options, host);
 
             if (typeReferences) {
-                const inferredRoot = getInferredTypesRoot(options, rootNames, host);
-                const containingFilename = combinePaths(inferredRoot, "__inferred type names__.ts");
+                // This containingFilename needs to match with the one used in managed-side
+                const containingFilename = combinePaths(host.getCurrentDirectory(), "__inferred type names__.ts");
                 const resolutions = resolveTypeReferenceDirectiveNamesWorker(typeReferences, containingFilename);
                 for (let i = 0; i < typeReferences.length; i++) {
                     processTypeReferenceDirective(typeReferences[i], resolutions[i]);
@@ -1214,8 +1213,8 @@ namespace ts {
         };
 
         verifyCompilerOptions();
-
-        performance.measure("Program", start);
+        performance.mark("afterProgram");
+        performance.measure("Program", "beforeProgram", "afterProgram");
 
         return program;
 
@@ -1242,10 +1241,10 @@ namespace ts {
             if (!classifiableNames) {
                 // Initialize a checker so that all our files are bound.
                 getTypeChecker();
-                classifiableNames = {};
+                classifiableNames = createMap<string>();
 
                 for (const sourceFile of files) {
-                    copyMap(sourceFile.classifiableNames, classifiableNames);
+                    copyProperties(sourceFile.classifiableNames, classifiableNames);
                 }
             }
 
@@ -1273,7 +1272,7 @@ namespace ts {
                 (oldOptions.maxNodeModuleJsDepth !== options.maxNodeModuleJsDepth) ||
                 !arrayIsEqualTo(oldOptions.typeRoots, oldOptions.typeRoots) ||
                 !arrayIsEqualTo(oldOptions.rootDirs, options.rootDirs) ||
-                !mapIsEqualTo(oldOptions.paths, options.paths)) {
+                !equalOwnProperties(oldOptions.paths, options.paths)) {
                 return false;
             }
 
@@ -1395,7 +1394,7 @@ namespace ts {
                 getSourceFile: program.getSourceFile,
                 getSourceFileByPath: program.getSourceFileByPath,
                 getSourceFiles: program.getSourceFiles,
-                isSourceFileFromExternalLibrary: (file: SourceFile) => !!lookUp(sourceFilesFoundSearchingNodeModules, file.path),
+                isSourceFileFromExternalLibrary: (file: SourceFile) => !!sourceFilesFoundSearchingNodeModules[file.path],
                 writeFile: writeFileCallback || (
                     (fileName, data, writeByteOrderMark, onError, sourceFiles) => host.writeFile(fileName, data, writeByteOrderMark, onError, sourceFiles)),
                 isEmitBlocked,
@@ -1458,14 +1457,15 @@ namespace ts {
             // checked is to not pass the file to getEmitResolver.
             const emitResolver = getDiagnosticsProducingTypeChecker().getEmitResolver((options.outFile || options.out) ? undefined : sourceFile);
 
-            const start = performance.mark();
+            performance.mark("beforeEmit");
 
             const emitResult = emitFiles(
                 emitResolver,
                 getEmitHost(writeFileCallback),
                 sourceFile);
 
-            performance.measure("Emit", start);
+            performance.mark("afterEmit");
+            performance.measure("Emit", "beforeEmit", "afterEmit");
             return emitResult;
         }
 
@@ -1932,7 +1932,7 @@ namespace ts {
 
                 // If the file was previously found via a node_modules search, but is now being processed as a root file,
                 // then everything it sucks in may also be marked incorrectly, and needs to be checked again.
-                if (file && lookUp(sourceFilesFoundSearchingNodeModules, file.path) && currentNodeModulesDepth == 0) {
+                if (file && sourceFilesFoundSearchingNodeModules[file.path] && currentNodeModulesDepth == 0) {
                     sourceFilesFoundSearchingNodeModules[file.path] = false;
                     if (!options.noResolve) {
                         processReferencedFiles(file, getDirectoryPath(fileName), isDefaultLib);
@@ -1943,7 +1943,7 @@ namespace ts {
                     processImportedModules(file, getDirectoryPath(fileName));
                 }
                 // See if we need to reprocess the imports due to prior skipped imports
-                else if (file && lookUp(modulesWithElidedImports, file.path)) {
+                else if (file && modulesWithElidedImports[file.path]) {
                     if (currentNodeModulesDepth < maxNodeModulesJsDepth) {
                         modulesWithElidedImports[file.path] = false;
                         processImportedModules(file, getDirectoryPath(fileName));
@@ -2010,15 +2010,16 @@ namespace ts {
         }
 
         function processTypeReferenceDirectives(file: SourceFile) {
-            const typeDirectives = map(file.typeReferenceDirectives, l => l.fileName);
+            const typeDirectives = map(file.typeReferenceDirectives, ref => ref.fileName.toLocaleLowerCase());
             const resolutions = resolveTypeReferenceDirectiveNamesWorker(typeDirectives, file.fileName);
 
             for (let i = 0; i < typeDirectives.length; i++) {
                 const ref = file.typeReferenceDirectives[i];
                 const resolvedTypeReferenceDirective = resolutions[i];
                 // store resolved type directive on the file
-                setResolvedTypeReferenceDirective(file, ref.fileName, resolvedTypeReferenceDirective);
-                processTypeReferenceDirective(ref.fileName, resolvedTypeReferenceDirective, file, ref.pos, ref.end);
+                const fileName = ref.fileName.toLocaleLowerCase();
+                setResolvedTypeReferenceDirective(file, fileName, resolvedTypeReferenceDirective);
+                processTypeReferenceDirective(fileName, resolvedTypeReferenceDirective, file, ref.pos, ref.end);
             }
         }
 
@@ -2043,7 +2044,7 @@ namespace ts {
                         const otherFileText = host.readFile(resolvedTypeReferenceDirective.resolvedFileName);
                         if (otherFileText !== getSourceFile(previousResolution.resolvedFileName).text) {
                             fileProcessingDiagnostics.add(createDiagnostic(refFile, refPos, refEnd,
-                                Diagnostics.Conflicting_library_definitions_for_0_found_at_1_and_2_Copy_the_correct_file_to_the_typings_folder_to_resolve_this_conflict,
+                                Diagnostics.Conflicting_definitions_for_0_found_at_1_and_2_Consider_installing_a_specific_version_of_this_library_to_resolve_the_conflict,
                                 typeReferenceDirective,
                                 resolvedTypeReferenceDirective.resolvedFileName,
                                 previousResolution.resolvedFileName
@@ -2083,7 +2084,7 @@ namespace ts {
         function processImportedModules(file: SourceFile, basePath: string) {
             collectExternalModuleReferences(file);
             if (file.imports.length || file.moduleAugmentations.length) {
-                file.resolvedModules = {};
+                file.resolvedModules = createMap<ResolvedModule>();
                 const moduleNames = map(concatenate(file.imports, file.moduleAugmentations), getTextOfLiteral);
                 const resolutions = resolveModuleNamesWorker(moduleNames, getNormalizedAbsolutePath(file.fileName, currentDirectory));
                 for (let i = 0; i < moduleNames.length; i++) {
diff --git a/src/compiler/scanner.ts b/src/compiler/scanner.ts
index 6dd84298008..c1431ca23ed 100644
--- a/src/compiler/scanner.ts
+++ b/src/compiler/scanner.ts
@@ -55,7 +55,7 @@ namespace ts {
         tryScan<T>(callback: () => T): T;
     }
 
-    const textToToken: Map<SyntaxKind> = {
+    const textToToken = createMap({
         "abstract": SyntaxKind.AbstractKeyword,
         "any": SyntaxKind.AnyKeyword,
         "as": SyntaxKind.AsKeyword,
@@ -179,7 +179,7 @@ namespace ts {
         "|=": SyntaxKind.BarEqualsToken,
         "^=": SyntaxKind.CaretEqualsToken,
         "@": SyntaxKind.AtToken,
-    };
+    });
 
     /*
         As per ECMAScript Language Specification 3th Edition, Section 7.6: Identifiers
@@ -274,9 +274,7 @@ namespace ts {
     function makeReverseMap(source: Map<number>): string[] {
         const result: string[] = [];
         for (const name in source) {
-            if (source.hasOwnProperty(name)) {
-                result[source[name]] = name;
-            }
+            result[source[name]] = name;
         }
         return result;
     }
diff --git a/src/compiler/sourcemap.ts b/src/compiler/sourcemap.ts
index 2d8c36a3d02..4046867faf1 100644
--- a/src/compiler/sourcemap.ts
+++ b/src/compiler/sourcemap.ts
@@ -46,6 +46,7 @@ namespace ts {
 
     export function createSourceMapWriter(host: EmitHost, writer: EmitTextWriter): SourceMapWriter {
         const compilerOptions = host.getCompilerOptions();
+        const extendedDiagnostics = compilerOptions.extendedDiagnostics;
         let currentSourceFile: SourceFile;
         let sourceMapDir: string; // The directory in which sourcemap will be
         let stopOverridingSpan = false;
@@ -240,7 +241,9 @@ namespace ts {
                 return;
             }
 
-            const start = performance.mark();
+            if (extendedDiagnostics) {
+                performance.mark("beforeSourcemap");
+            }
 
             const sourceLinePos = getLineAndCharacterOfPosition(currentSourceFile, pos);
 
@@ -282,7 +285,10 @@ namespace ts {
 
             updateLastEncodedAndRecordedSpans();
 
-            performance.measure("Source Map", start);
+            if (extendedDiagnostics) {
+                performance.mark("afterSourcemap");
+                performance.measure("Source Map", "beforeSourcemap", "afterSourcemap");
+            }
         }
 
         function getStartPos(range: TextRange) {
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 29ae2c60af1..350d75429b7 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -233,15 +233,15 @@ namespace ts {
             const useNonPollingWatchers = process.env["TSC_NONPOLLING_WATCHER"];
 
             function createWatchedFileSet() {
-                const dirWatchers: Map<DirectoryWatcher> = {};
+                const dirWatchers = createMap<DirectoryWatcher>();
                 // One file can have multiple watchers
-                const fileWatcherCallbacks: Map<FileWatcherCallback[]> = {};
+                const fileWatcherCallbacks = createMap<FileWatcherCallback[]>();
                 return { addFile, removeFile };
 
                 function reduceDirWatcherRefCountForFile(fileName: string) {
                     const dirName = getDirectoryPath(fileName);
-                    if (hasProperty(dirWatchers, dirName)) {
-                        const watcher = dirWatchers[dirName];
+                    const watcher = dirWatchers[dirName];
+                    if (watcher) {
                         watcher.referenceCount -= 1;
                         if (watcher.referenceCount <= 0) {
                             watcher.close();
@@ -251,13 +251,12 @@ namespace ts {
                 }
 
                 function addDirWatcher(dirPath: string): void {
-                    if (hasProperty(dirWatchers, dirPath)) {
-                        const watcher = dirWatchers[dirPath];
+                    let watcher = dirWatchers[dirPath];
+                    if (watcher) {
                         watcher.referenceCount += 1;
                         return;
                     }
-
-                    const watcher: DirectoryWatcher = _fs.watch(
+                    watcher = _fs.watch(
                         dirPath,
                         { persistent: true },
                         (eventName: string, relativeFileName: string) => fileEventHandler(eventName, relativeFileName, dirPath)
@@ -268,12 +267,7 @@ namespace ts {
                 }
 
                 function addFileWatcherCallback(filePath: string, callback: FileWatcherCallback): void {
-                    if (hasProperty(fileWatcherCallbacks, filePath)) {
-                        fileWatcherCallbacks[filePath].push(callback);
-                    }
-                    else {
-                        fileWatcherCallbacks[filePath] = [callback];
-                    }
+                    (fileWatcherCallbacks[filePath] || (fileWatcherCallbacks[filePath] = [])).push(callback);
                 }
 
                 function addFile(fileName: string, callback: FileWatcherCallback): WatchedFile {
@@ -289,8 +283,9 @@ namespace ts {
                 }
 
                 function removeFileWatcherCallback(filePath: string, callback: FileWatcherCallback) {
-                    if (hasProperty(fileWatcherCallbacks, filePath)) {
-                        const newCallbacks = copyListRemovingItem(callback, fileWatcherCallbacks[filePath]);
+                    const callbacks = fileWatcherCallbacks[filePath];
+                    if (callbacks) {
+                        const newCallbacks = copyListRemovingItem(callback, callbacks);
                         if (newCallbacks.length === 0) {
                             delete fileWatcherCallbacks[filePath];
                         }
@@ -306,7 +301,7 @@ namespace ts {
                         ? undefined
                         : ts.getNormalizedAbsolutePath(relativeFileName, baseDirPath);
                     // Some applications save a working file via rename operations
-                    if ((eventName === "change" || eventName === "rename") && hasProperty(fileWatcherCallbacks, fileName)) {
+                    if ((eventName === "change" || eventName === "rename") && fileWatcherCallbacks[fileName]) {
                         for (const fileCallback of fileWatcherCallbacks[fileName]) {
                             fileCallback(fileName);
                         }
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index 10538d0c009..8b445bcb626 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -122,11 +122,11 @@ namespace ts {
     const gutterSeparator = " ";
     const resetEscapeSequence = "\u001b[0m";
     const ellipsis = "...";
-    const categoryFormatMap: Map<string> = {
+    const categoryFormatMap = createMap<string>({
         [DiagnosticCategory.Warning]: yellowForegroundEscapeSequence,
         [DiagnosticCategory.Error]: redForegroundEscapeSequence,
         [DiagnosticCategory.Message]: blueForegroundEscapeSequence,
-    };
+    });
 
     function formatAndReset(text: string, formatStyle: string) {
         return formatStyle + text + resetEscapeSequence;
@@ -432,7 +432,7 @@ namespace ts {
             }
 
             // reset the cache of existing files
-            cachedExistingFiles = {};
+            cachedExistingFiles = createMap<boolean>();
 
             const compileResult = compile(rootFileNames, compilerOptions, compilerHost);
 
@@ -445,10 +445,9 @@ namespace ts {
         }
 
         function cachedFileExists(fileName: string): boolean {
-            if (hasProperty(cachedExistingFiles, fileName)) {
-                return cachedExistingFiles[fileName];
-            }
-            return cachedExistingFiles[fileName] = hostFileExists(fileName);
+            return fileName in cachedExistingFiles
+                ? cachedExistingFiles[fileName]
+                : cachedExistingFiles[fileName] = hostFileExists(fileName);
         }
 
         function getSourceFile(fileName: string, languageVersion: ScriptTarget, onError?: (message: string) => void) {
@@ -676,7 +675,7 @@ namespace ts {
         const usageColumn: string[] = []; // Things like "-d, --declaration" go in here.
         const descriptionColumn: string[] = [];
 
-        const optionsDescriptionMap: Map<string[]> = {};  // Map between option.description and list of option.type if it is a kind
+        const optionsDescriptionMap = createMap<string[]>();  // Map between option.description and list of option.type if it is a kind
 
         for (let i = 0; i < optsList.length; i++) {
             const option = optsList[i];
@@ -704,9 +703,10 @@ namespace ts {
                 description = getDiagnosticText(option.description);
                 const options: string[] = [];
                 const element = (<CommandLineOptionOfListType>option).element;
-                forEachKey(<Map<number | string>>element.type, key => {
+                const typeMap = <Map<number | string>>element.type;
+                for (const key in typeMap) {
                     options.push(`'${key}'`);
-                });
+                }
                 optionsDescriptionMap[description] = options;
             }
             else {
@@ -786,7 +786,7 @@ namespace ts {
         return;
 
         function serializeCompilerOptions(options: CompilerOptions): Map<string | number | boolean> {
-            const result: Map<string | number | boolean> = {};
+            const result = createMap<string | number | boolean>();
             const optionsNameMap = getOptionNameMap().optionNameMap;
 
             for (const name in options) {
@@ -812,9 +812,8 @@ namespace ts {
                                     // Enum
                                     const typeMap = <Map<number>>optionDefinition.type;
                                     for (const key in typeMap) {
-                                        if (hasProperty(typeMap, key)) {
-                                            if (typeMap[key] === value)
-                                                result[name] = key;
+                                        if (typeMap[key] === value) {
+                                            result[name] = key;
                                         }
                                     }
                                 }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 70bd3c3c070..b5e63ea6e07 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1,9 +1,13 @@
-
 namespace ts {
-    export interface Map<T> {
+
+    export interface MapLike<T> {
         [index: string]: T;
     }
 
+    export interface Map<T> extends MapLike<T> {
+        __mapBrand: any;
+    }
+
     // branded string type used to store absolute, normalized and canonicalized paths
     // arbitrary file name can be converted to Path via toPath function
     export type Path = string & { __pathBrand: any };
@@ -982,13 +986,19 @@ namespace ts {
         multiLine?: boolean;
     }
 
+    export type EntityNameExpression = Identifier | PropertyAccessEntityNameExpression;
+    export type EntityNameOrEntityNameExpression = EntityName | EntityNameExpression;
+
     // @kind(SyntaxKind.PropertyAccessExpression)
     export interface PropertyAccessExpression extends MemberExpression, Declaration {
         expression: LeftHandSideExpression;
         name: Identifier;
     }
-
-    export type IdentifierOrPropertyAccess = Identifier | PropertyAccessExpression;
+    /** Brand for a PropertyAccessExpression which, like a QualifiedName, consists of a sequence of identifiers separated by dots. */
+    export interface PropertyAccessEntityNameExpression extends PropertyAccessExpression {
+        _propertyAccessExpressionLikeQualifiedNameBrand?: any;
+        expression: EntityNameExpression;
+    }
 
     // @kind(SyntaxKind.ElementAccessExpression)
     export interface ElementAccessExpression extends MemberExpression {
@@ -1606,6 +1616,16 @@ namespace ts {
         antecedent: FlowNode;
     }
 
+    export type FlowType = Type | IncompleteType;
+
+    // Incomplete types occur during control flow analysis of loops. An IncompleteType
+    // is distinguished from a regular type by a flags value of zero. Incomplete type
+    // objects are internal to the getFlowTypeOfRefecence function and never escape it.
+    export interface IncompleteType {
+        flags: TypeFlags;  // No flags set
+        type: Type;        // The type marked incomplete
+    }
+
     export interface AmdDependency {
         path: string;
         name: string;
@@ -2021,7 +2041,7 @@ namespace ts {
         writeTypeOfExpression(expr: Expression, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         writeBaseConstructorTypeOfClass(node: ClassLikeDeclaration, enclosingDeclaration: Node, flags: TypeFormatFlags, writer: SymbolWriter): void;
         isSymbolAccessible(symbol: Symbol, enclosingDeclaration: Node, meaning: SymbolFlags): SymbolAccessibilityResult;
-        isEntityNameVisible(entityName: EntityName | Expression, enclosingDeclaration: Node): SymbolVisibilityResult;
+        isEntityNameVisible(entityName: EntityNameOrEntityNameExpression, enclosingDeclaration: Node): SymbolVisibilityResult;
         // Returns the constant value this property access resolves to, or 'undefined' for a non-constant
         getConstantValue(node: EnumMember | PropertyAccessExpression | ElementAccessExpression): number;
         getReferencedValueDeclaration(reference: Identifier): Declaration;
@@ -2030,7 +2050,7 @@ namespace ts {
         moduleExportsSomeValue(moduleReferenceExpression: Expression): boolean;
         isArgumentsLocalBinding(node: Identifier): boolean;
         getExternalModuleFileFromDeclaration(declaration: ImportEqualsDeclaration | ImportDeclaration | ExportDeclaration | ModuleDeclaration): SourceFile;
-        getTypeReferenceDirectivesForEntityName(name: EntityName | PropertyAccessExpression): string[];
+        getTypeReferenceDirectivesForEntityName(name: EntityNameOrEntityNameExpression): string[];
         getTypeReferenceDirectivesForSymbol(symbol: Symbol, meaning?: SymbolFlags): string[];
     }
 
@@ -2138,6 +2158,7 @@ namespace ts {
         /* @internal */ constEnumOnlyModule?: boolean; // True if module contains only const enums or other modules with only const enums
         /* @internal */ isReferenced?: boolean; // True if the symbol is referenced elsewhere
         /* @internal */ isReplaceableByMethod?: boolean; // Can this Javascript class property be replaced by a method symbol?
+        /* @internal */ isAssigned?: boolean;   // True if the symbol is a parameter with assignments
     }
 
     /* @internal */
@@ -2151,6 +2172,8 @@ namespace ts {
         mapper?: TypeMapper;                // Type mapper for instantiation alias
         referenced?: boolean;               // True if alias symbol has been referenced as a value
         containingType?: UnionOrIntersectionType; // Containing union or intersection type for synthetic property
+        hasCommonType?: boolean;            // True if constituents of synthetic property all have same type
+        isDiscriminantProperty?: boolean;   // True if discriminant synthetic property
         resolvedExports?: SymbolTable;      // Resolved exports of module
         exportsChecked?: boolean;           // True if exports of external module have been checked
         isDeclarationWithCollidingName?: boolean;    // True if symbol is block scoped redeclaration
@@ -2161,9 +2184,7 @@ namespace ts {
     /* @internal */
     export interface TransientSymbol extends Symbol, SymbolLinks { }
 
-    export interface SymbolTable {
-        [index: string]: Symbol;
-    }
+    export type SymbolTable = Map<Symbol>;
 
     /** Represents a "prefix*suffix" pattern. */
     /* @internal */
@@ -2190,23 +2211,24 @@ namespace ts {
         AsyncMethodWithSuper                = 0x00000800,  // An async method that reads a value from a member of 'super'.
         AsyncMethodWithSuperBinding         = 0x00001000,  // An async method that assigns a value to a member of 'super'.
         CaptureArguments                    = 0x00002000,  // Lexical 'arguments' used in body (for async functions)
-        EnumValuesComputed                  = 0x00004000, // Values for enum members have been computed, and any errors have been reported for them.
-        LexicalModuleMergesWithClass        = 0x00008000, // Instantiated lexical module declaration is merged with a previous class declaration.
-        LoopWithCapturedBlockScopedBinding  = 0x00010000, // Loop that contains block scoped variable captured in closure
-        CapturedBlockScopedBinding          = 0x00020000, // Block-scoped binding that is captured in some function
-        BlockScopedBindingInLoop            = 0x00040000, // Block-scoped binding with declaration nested inside iteration statement
-        ClassWithBodyScopedClassBinding     = 0x00080000, // Decorated class that contains a binding to itself inside of the class body.
-        BodyScopedClassBinding              = 0x00100000, // Binding to a decorated class inside of the class's body.
-        NeedsLoopOutParameter               = 0x00200000, // Block scoped binding whose value should be explicitly copied outside of the converted loop
+        EnumValuesComputed                  = 0x00004000,  // Values for enum members have been computed, and any errors have been reported for them.
+        LexicalModuleMergesWithClass        = 0x00008000,  // Instantiated lexical module declaration is merged with a previous class declaration.
+        LoopWithCapturedBlockScopedBinding  = 0x00010000,  // Loop that contains block scoped variable captured in closure
+        CapturedBlockScopedBinding          = 0x00020000,  // Block-scoped binding that is captured in some function
+        BlockScopedBindingInLoop            = 0x00040000,  // Block-scoped binding with declaration nested inside iteration statement
+        ClassWithBodyScopedClassBinding     = 0x00080000,  // Decorated class that contains a binding to itself inside of the class body.
+        BodyScopedClassBinding              = 0x00100000,  // Binding to a decorated class inside of the class's body.
+        NeedsLoopOutParameter               = 0x00200000,  // Block scoped binding whose value should be explicitly copied outside of the converted loop
+        AssignmentsMarked                   = 0x00400000,  // Parameter assignments have been marked
     }
 
     /* @internal */
     export interface NodeLinks {
+        flags?: NodeCheckFlags;           // Set of flags specific to Node
         resolvedType?: Type;              // Cached type of type node
         resolvedSignature?: Signature;    // Cached signature of signature node or call expression
         resolvedSymbol?: Symbol;          // Cached name resolution result
         resolvedIndexInfo?: IndexInfo;    // Cached indexing info resolution result
-        flags?: NodeCheckFlags;           // Set of flags specific to Node
         enumMemberValue?: number;         // Constant value of enum member
         isVisible?: boolean;              // Is this node visible
         hasReportedStatementInAmbientContext?: boolean;  // Cache boolean if we report statements in ambient context
@@ -2360,6 +2382,7 @@ namespace ts {
 
     export interface TupleType extends ObjectType {
         elementTypes: Type[];  // Element types
+        thisType?: Type;       // This-type of tuple (only needed for tuples that are constraints of type parameters)
     }
 
     export interface UnionOrIntersectionType extends Type {
@@ -2546,7 +2569,7 @@ namespace ts {
     }
 
     export type RootPaths = string[];
-    export type PathSubstitutions = Map<string[]>;
+    export type PathSubstitutions = MapLike<string[]>;
     export type TsConfigOnlyOptions = RootPaths | PathSubstitutions;
 
     export type CompilerOptionsValue = string | number | boolean | (string | number)[] | TsConfigOnlyOptions;
@@ -2706,7 +2729,7 @@ namespace ts {
         fileNames: string[];
         raw?: any;
         errors: Diagnostic[];
-        wildcardDirectories?: Map<WatchDirectoryFlags>;
+        wildcardDirectories?: MapLike<WatchDirectoryFlags>;
     }
 
     export const enum WatchDirectoryFlags {
@@ -2716,7 +2739,7 @@ namespace ts {
 
     export interface ExpandResult {
         fileNames: string[];
-        wildcardDirectories: Map<WatchDirectoryFlags>;
+        wildcardDirectories: MapLike<WatchDirectoryFlags>;
     }
 
     /* @internal */
@@ -2901,6 +2924,7 @@ namespace ts {
         directoryExists?(directoryName: string): boolean;
         realpath?(path: string): string;
         getCurrentDirectory?(): string;
+        getDirectories?(path: string): string[];
     }
 
     export interface ResolvedModule {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 4ae0005e9c5..0a1f43203ce 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -87,25 +87,6 @@ namespace ts {
         return node.end - node.pos;
     }
 
-    export function mapIsEqualTo<T>(map1: Map<T>, map2: Map<T>): boolean {
-        if (!map1 || !map2) {
-            return map1 === map2;
-        }
-        return containsAll(map1, map2) && containsAll(map2, map1);
-    }
-
-    function containsAll<T>(map: Map<T>, other: Map<T>): boolean {
-        for (const key in map) {
-            if (!hasProperty(map, key)) {
-                continue;
-            }
-            if (!hasProperty(other, key) || map[key] !== other[key]) {
-                return false;
-            }
-        }
-        return true;
-    }
-
     export function arrayIsEqualTo<T>(array1: T[], array2: T[], equaler?: (a: T, b: T) => boolean): boolean {
         if (!array1 || !array2) {
             return array1 === array2;
@@ -126,7 +107,7 @@ namespace ts {
     }
 
     export function hasResolvedModule(sourceFile: SourceFile, moduleNameText: string): boolean {
-        return sourceFile.resolvedModules && hasProperty(sourceFile.resolvedModules, moduleNameText);
+        return !!(sourceFile.resolvedModules && sourceFile.resolvedModules[moduleNameText]);
     }
 
     export function getResolvedModule(sourceFile: SourceFile, moduleNameText: string): ResolvedModule {
@@ -135,7 +116,7 @@ namespace ts {
 
     export function setResolvedModule(sourceFile: SourceFile, moduleNameText: string, resolvedModule: ResolvedModule): void {
         if (!sourceFile.resolvedModules) {
-            sourceFile.resolvedModules = {};
+            sourceFile.resolvedModules = createMap<ResolvedModule>();
         }
 
         sourceFile.resolvedModules[moduleNameText] = resolvedModule;
@@ -143,7 +124,7 @@ namespace ts {
 
     export function setResolvedTypeReferenceDirective(sourceFile: SourceFile, typeReferenceDirectiveName: string, resolvedTypeReferenceDirective: ResolvedTypeReferenceDirective): void {
         if (!sourceFile.resolvedTypeReferenceDirectiveNames) {
-            sourceFile.resolvedTypeReferenceDirectiveNames = {};
+            sourceFile.resolvedTypeReferenceDirectiveNames = createMap<ResolvedTypeReferenceDirective>();
         }
 
         sourceFile.resolvedTypeReferenceDirectiveNames[typeReferenceDirectiveName] = resolvedTypeReferenceDirective;
@@ -166,7 +147,7 @@ namespace ts {
         }
         for (let i = 0; i < names.length; i++) {
             const newResolution = newResolutions[i];
-            const oldResolution = oldResolutions && hasProperty(oldResolutions, names[i]) ? oldResolutions[names[i]] : undefined;
+            const oldResolution = oldResolutions && oldResolutions[names[i]];
             const changed =
                 oldResolution
                     ? !newResolution || !comparer(oldResolution, newResolution)
@@ -1033,14 +1014,14 @@ namespace ts {
             && (<PropertyAccessExpression | ElementAccessExpression>node).expression.kind === SyntaxKind.SuperKeyword;
     }
 
-
-    export function getEntityNameFromTypeNode(node: TypeNode): EntityName | Expression {
+    export function getEntityNameFromTypeNode(node: TypeNode): EntityNameOrEntityNameExpression {
         if (node) {
             switch (node.kind) {
                 case SyntaxKind.TypeReference:
                     return (<TypeReferenceNode>node).typeName;
                 case SyntaxKind.ExpressionWithTypeArguments:
-                    return (<ExpressionWithTypeArguments>node).expression;
+                    Debug.assert(isEntityNameExpression((<ExpressionWithTypeArguments>node).expression));
+                    return <EntityNameExpression>(<ExpressionWithTypeArguments>node).expression;
                 case SyntaxKind.Identifier:
                 case SyntaxKind.QualifiedName:
                     return (<EntityName><Node>node);
@@ -1569,6 +1550,7 @@ namespace ts {
             case SyntaxKind.MethodSignature:
             case SyntaxKind.ModuleDeclaration:
             case SyntaxKind.NamespaceImport:
+            case SyntaxKind.NamespaceExportDeclaration:
             case SyntaxKind.Parameter:
             case SyntaxKind.PropertyAssignment:
             case SyntaxKind.PropertyDeclaration:
@@ -1693,8 +1675,8 @@ namespace ts {
     // import * as <symbol> from ...
     // import { x as <symbol> } from ...
     // export { x as <symbol> } from ...
-    // export = ...
-    // export default ...
+    // export = <EntityNameExpression>
+    // export default <EntityNameExpression>
     export function isAliasSymbolDeclaration(node: Node): boolean {
         return node.kind === SyntaxKind.ImportEqualsDeclaration ||
             node.kind === SyntaxKind.NamespaceExportDeclaration ||
@@ -1702,7 +1684,11 @@ namespace ts {
             node.kind === SyntaxKind.NamespaceImport ||
             node.kind === SyntaxKind.ImportSpecifier ||
             node.kind === SyntaxKind.ExportSpecifier ||
-            node.kind === SyntaxKind.ExportAssignment && (<ExportAssignment>node).expression.kind === SyntaxKind.Identifier;
+            node.kind === SyntaxKind.ExportAssignment && exportAssignmentIsAlias(<ExportAssignment>node);
+    }
+
+    export function exportAssignmentIsAlias(node: ExportAssignment): boolean {
+        return isEntityNameExpression(node.expression);
     }
 
     export function getClassExtendsHeritageClauseElement(node: ClassLikeDeclaration | InterfaceDeclaration) {
@@ -1965,7 +1951,7 @@ namespace ts {
 
     export function createDiagnosticCollection(): DiagnosticCollection {
         let nonFileDiagnostics: Diagnostic[] = [];
-        const fileDiagnostics: Map<Diagnostic[]> = {};
+        const fileDiagnostics = createMap<Diagnostic[]>();
 
         let diagnosticsModified = false;
         let modificationCount = 0;
@@ -1983,12 +1969,11 @@ namespace ts {
         }
 
         function reattachFileDiagnostics(newFile: SourceFile): void {
-            if (!hasProperty(fileDiagnostics, newFile.fileName)) {
-                return;
-            }
-
-            for (const diagnostic of fileDiagnostics[newFile.fileName]) {
-                diagnostic.file = newFile;
+            const diagnostics = fileDiagnostics[newFile.fileName];
+            if (diagnostics) {
+                for (const diagnostic of diagnostics) {
+                    diagnostic.file = newFile;
+                }
             }
         }
 
@@ -2029,9 +2014,7 @@ namespace ts {
             forEach(nonFileDiagnostics, pushDiagnostic);
 
             for (const key in fileDiagnostics) {
-                if (hasProperty(fileDiagnostics, key)) {
-                    forEach(fileDiagnostics[key], pushDiagnostic);
-                }
+                forEach(fileDiagnostics[key], pushDiagnostic);
             }
 
             return sortAndDeduplicateDiagnostics(allDiagnostics);
@@ -2046,9 +2029,7 @@ namespace ts {
             nonFileDiagnostics = sortAndDeduplicateDiagnostics(nonFileDiagnostics);
 
             for (const key in fileDiagnostics) {
-                if (hasProperty(fileDiagnostics, key)) {
-                    fileDiagnostics[key] = sortAndDeduplicateDiagnostics(fileDiagnostics[key]);
-                }
+                fileDiagnostics[key] = sortAndDeduplicateDiagnostics(fileDiagnostics[key]);
             }
         }
     }
@@ -2059,7 +2040,7 @@ namespace ts {
     // the map below must be updated. Note that this regexp *does not* include the 'delete' character.
     // There is no reason for this other than that JSON.stringify does not handle it either.
     const escapedCharsRegExp = /[\\\"\u0000-\u001f\t\v\f\b\r\n\u2028\u2029\u0085]/g;
-    const escapedCharsMap: Map<string> = {
+    const escapedCharsMap = createMap({
         "\0": "\\0",
         "\t": "\\t",
         "\v": "\\v",
@@ -2072,7 +2053,7 @@ namespace ts {
         "\u2028": "\\u2028", // lineSeparator
         "\u2029": "\\u2029", // paragraphSeparator
         "\u0085": "\\u0085"  // nextLine
-    };
+    });
 
 
     /**
@@ -2680,22 +2661,9 @@ namespace ts {
             isClassLike(node.parent.parent);
     }
 
-    // Returns false if this heritage clause element's expression contains something unsupported
-    // (i.e. not a name or dotted name).
-    export function isSupportedExpressionWithTypeArguments(node: ExpressionWithTypeArguments): boolean {
-        return isSupportedExpressionWithTypeArgumentsRest(node.expression);
-    }
-
-    function isSupportedExpressionWithTypeArgumentsRest(node: Expression): boolean {
-        if (node.kind === SyntaxKind.Identifier) {
-            return true;
-        }
-        else if (isPropertyAccessExpression(node)) {
-            return isSupportedExpressionWithTypeArgumentsRest(node.expression);
-        }
-        else {
-            return false;
-        }
+    export function isEntityNameExpression(node: Expression): node is EntityNameExpression {
+        return node.kind === SyntaxKind.Identifier ||
+            node.kind === SyntaxKind.PropertyAccessExpression && isEntityNameExpression((<PropertyAccessExpression>node).expression);
     }
 
     export function isRightSideOfQualifiedNameOrPropertyAccess(node: Node) {
@@ -2805,7 +2773,7 @@ namespace ts {
     }
 
     function stringifyObject(value: any) {
-        return `{${reduceProperties(value, stringifyProperty, "")}}`;
+        return `{${reduceOwnProperties(value, stringifyProperty, "")}}`;
     }
 
     function stringifyProperty(memo: string, value: any, key: string) {
diff --git a/src/harness/compilerRunner.ts b/src/harness/compilerRunner.ts
index ecdc18394b0..66396293dc2 100644
--- a/src/harness/compilerRunner.ts
+++ b/src/harness/compilerRunner.ts
@@ -52,7 +52,7 @@ class CompilerBaselineRunner extends RunnerBase {
     private makeUnitName(name: string, root: string) {
         const path = ts.toPath(name, root, (fileName) => Harness.Compiler.getCanonicalFileName(fileName));
         const pathStart = ts.toPath(Harness.IO.getCurrentDirectory(), "", (fileName) => Harness.Compiler.getCanonicalFileName(fileName));
-        return path.replace(pathStart, "/");
+        return pathStart ? path.replace(pathStart, "/") : path;
     };
 
     public checkTestCodeOutput(fileName: string) {
@@ -291,8 +291,8 @@ class CompilerBaselineRunner extends RunnerBase {
 
                 const fullWalker = new TypeWriterWalker(program, /*fullTypeCheck*/ true);
 
-                const fullResults: ts.Map<TypeWriterResult[]> = {};
-                const pullResults: ts.Map<TypeWriterResult[]> = {};
+                const fullResults = ts.createMap<TypeWriterResult[]>();
+                const pullResults = ts.createMap<TypeWriterResult[]>();
 
                 for (const sourceFile of allFiles) {
                     fullResults[sourceFile.unitName] = fullWalker.getTypeAndSymbols(sourceFile.unitName);
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index a42abbbc609..b5fa53763cb 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -95,14 +95,14 @@ namespace FourSlash {
 
     export import IndentStyle = ts.IndentStyle;
 
-    const entityMap: ts.Map<string> = {
+    const entityMap = ts.createMap({
         "&": "&amp;",
         "\"": "&quot;",
         "'": "&#39;",
         "/": "&#47;",
         "<": "&lt;",
         ">": "&gt;"
-    };
+    });
 
     export function escapeXmlAttributeValue(s: string) {
         return s.replace(/[&<>"'\/]/g, ch => entityMap[ch]);
@@ -204,7 +204,7 @@ namespace FourSlash {
 
         public formatCodeOptions: ts.FormatCodeOptions;
 
-        private inputFiles: ts.Map<string> = {};  // Map between inputFile's fileName and its content for easily looking up when resolving references
+        private inputFiles = ts.createMap<string>();  // Map between inputFile's fileName and its content for easily looking up when resolving references
 
         // Add input file which has matched file name with the given reference-file path.
         // This is necessary when resolveReference flag is specified
@@ -249,6 +249,7 @@ namespace FourSlash {
             if (compilationOptions.typeRoots) {
                 compilationOptions.typeRoots = compilationOptions.typeRoots.map(p => ts.getNormalizedAbsolutePath(p, this.basePath));
             }
+            compilationOptions.skipDefaultLibCheck = true;
 
             const languageServiceAdapter = this.getLanguageServiceAdapter(testType, this.cancellationToken, compilationOptions);
             this.languageServiceAdapterHost = languageServiceAdapter.getHost();
@@ -300,11 +301,11 @@ namespace FourSlash {
             }
             else {
                 // resolveReference file-option is not specified then do not resolve any files and include all inputFiles
-                ts.forEachKey(this.inputFiles, fileName => {
+                for (const fileName in this.inputFiles) {
                     if (!Harness.isDefaultLibraryFile(fileName)) {
                         this.languageServiceAdapterHost.addScript(fileName, this.inputFiles[fileName], /*isRootFile*/ true);
                     }
-                });
+                }
                 this.languageServiceAdapterHost.addScript(Harness.Compiler.defaultLibFileName,
                     Harness.Compiler.getDefaultLibrarySourceFile().text, /*isRootFile*/ false);
             }
@@ -376,7 +377,7 @@ namespace FourSlash {
         public verifyErrorExistsBetweenMarkers(startMarkerName: string, endMarkerName: string, negative: boolean) {
             const startMarker = this.getMarkerByName(startMarkerName);
             const endMarker = this.getMarkerByName(endMarkerName);
-            const predicate = function (errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
+            const predicate = function(errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
                 return ((errorMinChar === startPos) && (errorLimChar === endPos)) ? true : false;
             };
 
@@ -428,12 +429,12 @@ namespace FourSlash {
             let predicate: (errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) => boolean;
 
             if (after) {
-                predicate = function (errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
+                predicate = function(errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
                     return ((errorMinChar >= startPos) && (errorLimChar >= startPos)) ? true : false;
                 };
             }
             else {
-                predicate = function (errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
+                predicate = function(errorMinChar: number, errorLimChar: number, startPos: number, endPos: number) {
                     return ((errorMinChar <= startPos) && (errorLimChar <= startPos)) ? true : false;
                 };
             }
@@ -458,7 +459,7 @@ namespace FourSlash {
                 endPos = endMarker.position;
             }
 
-            errors.forEach(function (error: ts.Diagnostic) {
+            errors.forEach(function(error: ts.Diagnostic) {
                 if (predicate(error.start, error.start + error.length, startPos, endPos)) {
                     exists = true;
                 }
@@ -475,7 +476,7 @@ namespace FourSlash {
                 Harness.IO.log("Unexpected error(s) found.  Error list is:");
             }
 
-            errors.forEach(function (error: ts.Diagnostic) {
+            errors.forEach(function(error: ts.Diagnostic) {
                 Harness.IO.log("  minChar: " + error.start +
                     ", limChar: " + (error.start + error.length) +
                     ", message: " + ts.flattenDiagnosticMessageText(error.messageText, Harness.IO.newLine()) + "\n");
@@ -593,9 +594,9 @@ namespace FourSlash {
 
         public noItemsWithSameNameButDifferentKind(): void {
             const completions = this.getCompletionListAtCaret();
-            const uniqueItems: ts.Map<string> = {};
+            const uniqueItems = ts.createMap<string>();
             for (const item of completions.entries) {
-                if (!ts.hasProperty(uniqueItems, item.name)) {
+                if (!(item.name in uniqueItems)) {
                     uniqueItems[item.name] = item.kind;
                 }
                 else {
@@ -773,7 +774,7 @@ namespace FourSlash {
         }
 
         public verifyRangesWithSameTextReferenceEachOther() {
-            ts.forEachValue(this.rangesByText(), ranges => this.verifyRangesReferenceEachOther(ranges));
+            ts.forEachProperty(this.rangesByText(), ranges => this.verifyRangesReferenceEachOther(ranges));
         }
 
         private verifyReferencesWorker(references: ts.ReferenceEntry[], fileName: string, start: number, end: number, isWriteAccess?: boolean, isDefinition?: boolean) {
@@ -1348,14 +1349,7 @@ namespace FourSlash {
         }
 
         // Enters lines of text at the current caret position
-        public type(text: string) {
-            return this.typeHighFidelity(text);
-        }
-
-        // Enters lines of text at the current caret position, invoking
-        // language service APIs to mimic Visual Studio's behavior
-        // as much as possible
-        private typeHighFidelity(text: string) {
+        public type(text: string, highFidelity = false) {
             let offset = this.currentCaretPosition;
             const prevChar = " ";
             const checkCadence = (text.length >> 2) + 1;
@@ -1364,24 +1358,26 @@ namespace FourSlash {
                 // Make the edit
                 const ch = text.charAt(i);
                 this.languageServiceAdapterHost.editScript(this.activeFile.fileName, offset, offset, ch);
-                this.languageService.getBraceMatchingAtPosition(this.activeFile.fileName, offset);
+                if (highFidelity) {
+                    this.languageService.getBraceMatchingAtPosition(this.activeFile.fileName, offset);
+                }
 
                 this.updateMarkersForEdit(this.activeFile.fileName, offset, offset, ch);
                 offset++;
 
-                if (ch === "(" || ch === ",") {
-                    /* Signature help*/
-                    this.languageService.getSignatureHelpItems(this.activeFile.fileName, offset);
-                }
-                else if (prevChar === " " && /A-Za-z_/.test(ch)) {
-                    /* Completions */
-                    this.languageService.getCompletionsAtPosition(this.activeFile.fileName, offset);
-                }
+                if (highFidelity) {
+                    if (ch === "(" || ch === ",") {
+                        /* Signature help*/
+                        this.languageService.getSignatureHelpItems(this.activeFile.fileName, offset);
+                    }
+                    else if (prevChar === " " && /A-Za-z_/.test(ch)) {
+                        /* Completions */
+                        this.languageService.getCompletionsAtPosition(this.activeFile.fileName, offset);
+                    }
 
-                if (i % checkCadence === 0) {
-                    this.checkPostEditInvariants();
-                    // this.languageService.getSyntacticDiagnostics(this.activeFile.fileName);
-                    // this.languageService.getSemanticDiagnostics(this.activeFile.fileName);
+                    if (i % checkCadence === 0) {
+                        this.checkPostEditInvariants();
+                    }
                 }
 
                 // Handle post-keystroke formatting
@@ -1389,14 +1385,12 @@ namespace FourSlash {
                     const edits = this.languageService.getFormattingEditsAfterKeystroke(this.activeFile.fileName, offset, ch, this.formatCodeOptions);
                     if (edits.length) {
                         offset += this.applyEdits(this.activeFile.fileName, edits, /*isFormattingEdit*/ true);
-                        // this.checkPostEditInvariants();
                     }
                 }
             }
 
             // Move the caret to wherever we ended up
             this.currentCaretPosition = offset;
-
             this.fixCaretPosition();
             this.checkPostEditInvariants();
         }
@@ -1415,7 +1409,6 @@ namespace FourSlash {
                 const edits = this.languageService.getFormattingEditsForRange(this.activeFile.fileName, start, offset, this.formatCodeOptions);
                 if (edits.length) {
                     offset += this.applyEdits(this.activeFile.fileName, edits, /*isFormattingEdit*/ true);
-                    this.checkPostEditInvariants();
                 }
             }
 
@@ -1639,10 +1632,11 @@ namespace FourSlash {
         }
 
         public rangesByText(): ts.Map<Range[]> {
-            const result: ts.Map<Range[]> = {};
+            const result = ts.createMap<Range[]>();
             for (const range of this.getRanges()) {
                 const text = this.rangeText(range);
-                (ts.getProperty(result, text) || (result[text] = [])).push(range);
+                const ranges = result[text] || (result[text] = []);
+                ranges.push(range);
             }
             return result;
         }
@@ -1897,7 +1891,7 @@ namespace FourSlash {
 
         public verifyBraceCompletionAtPosition(negative: boolean, openingBrace: string) {
 
-            const openBraceMap: ts.Map<ts.CharacterCodes> = {
+            const openBraceMap = ts.createMap<ts.CharacterCodes>({
                 "(": ts.CharacterCodes.openParen,
                 "{": ts.CharacterCodes.openBrace,
                 "[": ts.CharacterCodes.openBracket,
@@ -1905,7 +1899,7 @@ namespace FourSlash {
                 '"': ts.CharacterCodes.doubleQuote,
                 "`": ts.CharacterCodes.backtick,
                 "<": ts.CharacterCodes.lessThan
-            };
+            });
 
             const charCode = openBraceMap[openingBrace];
 
@@ -2269,40 +2263,12 @@ namespace FourSlash {
     export function runFourSlashTestContent(basePath: string, testType: FourSlashTestType, content: string, fileName: string): void {
         // Parse out the files and their metadata
         const testData = parseTestData(basePath, content, fileName);
-
         const state = new TestState(basePath, testType, testData);
-
-        let result = "";
-        const fourslashFile: Harness.Compiler.TestFile = {
-            unitName: Harness.Compiler.fourslashFileName,
-            content: undefined,
-        };
-        const testFile: Harness.Compiler.TestFile = {
-            unitName: fileName,
-            content: content
-        };
-
-        const host = Harness.Compiler.createCompilerHost(
-            [fourslashFile, testFile],
-            (fn, contents) => result = contents,
-            ts.ScriptTarget.Latest,
-            Harness.IO.useCaseSensitiveFileNames(),
-            Harness.IO.getCurrentDirectory());
-
-        const program = ts.createProgram([Harness.Compiler.fourslashFileName, fileName], { outFile: "fourslashTestOutput.js", noResolve: true, target: ts.ScriptTarget.ES3 }, host);
-
-        const sourceFile = host.getSourceFile(fileName, ts.ScriptTarget.ES3);
-
-        const diagnostics = ts.getPreEmitDiagnostics(program, sourceFile);
-        if (diagnostics.length > 0) {
-            throw new Error(`Error compiling ${fileName}: ` +
-                diagnostics.map(e => ts.flattenDiagnosticMessageText(e.messageText, Harness.IO.newLine())).join("\r\n"));
+        const output = ts.transpileModule(content, { reportDiagnostics: true });
+        if (output.diagnostics.length > 0) {
+            throw new Error(`Syntax error in ${basePath}: ${output.diagnostics[0].messageText}`);
         }
-
-        program.emit(sourceFile);
-
-        ts.Debug.assert(!!result);
-        runCode(result, state);
+        runCode(output.outputText, state);
     }
 
     function runCode(code: string, state: TestState): void {
@@ -2395,13 +2361,14 @@ ${code}
                 // Comment line, check for global/file @options and record them
                 const match = optionRegex.exec(line.substr(2));
                 if (match) {
-                    const fileMetadataNamesIndex = fileMetadataNames.indexOf(match[1]);
+                    const [key, value] = match.slice(1);
+                    const fileMetadataNamesIndex = fileMetadataNames.indexOf(key);
                     if (fileMetadataNamesIndex === -1) {
                         // Check if the match is already existed in the global options
-                        if (globalOptions[match[1]] !== undefined) {
-                            throw new Error("Global Option : '" + match[1] + "' is already existed");
+                        if (globalOptions[key] !== undefined) {
+                            throw new Error(`Global option '${key}' already exists`);
                         }
-                        globalOptions[match[1]] = match[2];
+                        globalOptions[key] = value;
                     }
                     else {
                         if (fileMetadataNamesIndex === fileMetadataNames.indexOf(metadataOptionNames.fileName)) {
@@ -2416,12 +2383,12 @@ ${code}
                                 resetLocalData();
                             }
 
-                            currentFileName = basePath + "/" + match[2];
-                            currentFileOptions[match[1]] = match[2];
+                            currentFileName = basePath + "/" + value;
+                            currentFileOptions[key] = value;
                         }
                         else {
                             // Add other fileMetadata flag
-                            currentFileOptions[match[1]] = match[2];
+                            currentFileOptions[key] = value;
                         }
                     }
                 }
@@ -2509,7 +2476,7 @@ ${code}
         }
 
         const marker: Marker = {
-            fileName: fileName,
+            fileName,
             position: location.position,
             data: markerValue
         };
@@ -2526,7 +2493,7 @@ ${code}
 
     function recordMarker(fileName: string, location: LocationInformation, name: string, markerMap: MarkerMap, markers: Marker[]): Marker {
         const marker: Marker = {
-            fileName: fileName,
+            fileName,
             position: location.position
         };
 
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index f27e7e1c174..e3bcb001863 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -750,7 +750,7 @@ namespace Harness {
 
             export function readDirectory(path: string, extension?: string[], exclude?: string[], include?: string[]) {
                 const fs = new Utils.VirtualFileSystem(path, useCaseSensitiveFileNames());
-                for (const file in listFiles(path)) {
+                for (const file of listFiles(path)) {
                     fs.addFile(file);
                 }
                 return ts.matchFiles(path, extension, exclude, include, useCaseSensitiveFileNames(), getCurrentDirectory(), path => {
@@ -848,9 +848,9 @@ namespace Harness {
         export const defaultLibFileName = "lib.d.ts";
         export const es2015DefaultLibFileName = "lib.es2015.d.ts";
 
-        const libFileNameSourceFileMap: ts.Map<ts.SourceFile> = {
+        const libFileNameSourceFileMap=  ts.createMap<ts.SourceFile>({
             [defaultLibFileName]: createSourceFileAndAssertInvariants(defaultLibFileName, IO.readFile(libFolder + "lib.es5.d.ts"), /*languageVersion*/ ts.ScriptTarget.Latest)
-        };
+        });
 
         export function getDefaultLibrarySourceFile(fileName = defaultLibFileName): ts.SourceFile {
             if (!isDefaultLibraryFile(fileName)) {
@@ -1005,13 +1005,13 @@ namespace Harness {
         let optionsIndex: ts.Map<ts.CommandLineOption>;
         function getCommandLineOption(name: string): ts.CommandLineOption {
             if (!optionsIndex) {
-                optionsIndex = {};
+                optionsIndex = ts.createMap<ts.CommandLineOption>();
                 const optionDeclarations = harnessOptionDeclarations.concat(ts.optionDeclarations);
                 for (const option of optionDeclarations) {
                     optionsIndex[option.name.toLowerCase()] = option;
                 }
             }
-            return ts.lookUp(optionsIndex, name.toLowerCase());
+            return optionsIndex[name.toLowerCase()];
         }
 
         export function setCompilerOptionsFromHarnessSetting(settings: Harness.TestCaseParser.CompilerSettings, options: ts.CompilerOptions & HarnessOptions): void {
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index d7ed04b627f..94124432780 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -123,7 +123,7 @@ namespace Harness.LanguageService {
     }
 
     export class LanguageServiceAdapterHost {
-        protected fileNameToScript: ts.Map<ScriptInfo> = {};
+        protected fileNameToScript = ts.createMap<ScriptInfo>();
 
         constructor(protected cancellationToken = DefaultHostCancellationToken.Instance,
                     protected settings = ts.getDefaultCompilerOptions()) {
@@ -135,7 +135,7 @@ namespace Harness.LanguageService {
 
         public getFilenames(): string[] {
             const fileNames: string[] = [];
-            ts.forEachValue(this.fileNameToScript, (scriptInfo) => {
+            ts.forEachProperty(this.fileNameToScript, (scriptInfo) => {
                 if (scriptInfo.isRootFile) {
                     // only include root files here
                     // usually it means that we won't include lib.d.ts in the list of root files so it won't mess the computation of compilation root dir.
@@ -146,7 +146,7 @@ namespace Harness.LanguageService {
         }
 
         public getScriptInfo(fileName: string): ScriptInfo {
-            return ts.lookUp(this.fileNameToScript, fileName);
+            return this.fileNameToScript[fileName];
         }
 
         public addScript(fileName: string, content: string, isRootFile: boolean): void {
@@ -235,7 +235,7 @@ namespace Harness.LanguageService {
                 this.getModuleResolutionsForFile = (fileName) => {
                     const scriptInfo = this.getScriptInfo(fileName);
                     const preprocessInfo = ts.preProcessFile(scriptInfo.content, /*readImportFiles*/ true);
-                    const imports: ts.Map<string> = {};
+                    const imports = ts.createMap<string>();
                     for (const module of preprocessInfo.importedFiles) {
                         const resolutionInfo = ts.resolveModuleName(module.fileName, fileName, compilerOptions, moduleResolutionHost);
                         if (resolutionInfo.resolvedModule) {
@@ -248,7 +248,7 @@ namespace Harness.LanguageService {
                     const scriptInfo = this.getScriptInfo(fileName);
                     if (scriptInfo) {
                         const preprocessInfo = ts.preProcessFile(scriptInfo.content, /*readImportFiles*/ false);
-                        const resolutions: ts.Map<ts.ResolvedTypeReferenceDirective> = {};
+                        const resolutions = ts.createMap<ts.ResolvedTypeReferenceDirective>();
                         const settings = this.nativeHost.getCompilationSettings();
                         for (const typeReferenceDirective of preprocessInfo.typeReferenceDirectives) {
                             const resolutionInfo = ts.resolveTypeReferenceDirective(typeReferenceDirective.fileName, fileName, settings, moduleResolutionHost);
diff --git a/src/harness/projectsRunner.ts b/src/harness/projectsRunner.ts
index 038b2dbe359..76f042834bb 100644
--- a/src/harness/projectsRunner.ts
+++ b/src/harness/projectsRunner.ts
@@ -253,18 +253,15 @@ class ProjectRunner extends RunnerBase {
                     moduleResolution: ts.ModuleResolutionKind.Classic, // currently all tests use classic module resolution kind, this will change in the future
                 };
                 // Set the values specified using json
-                const optionNameMap: ts.Map<ts.CommandLineOption> = {};
-                ts.forEach(ts.optionDeclarations, option => {
-                    optionNameMap[option.name] = option;
-                });
+                const optionNameMap = ts.arrayToMap(ts.optionDeclarations, option => option.name);
                 for (const name in testCase) {
-                    if (name !== "mapRoot" && name !== "sourceRoot" && ts.hasProperty(optionNameMap, name)) {
+                    if (name !== "mapRoot" && name !== "sourceRoot" && name in optionNameMap) {
                         const option = optionNameMap[name];
                         const optType = option.type;
                         let value = <any>testCase[name];
                         if (typeof optType !== "string") {
                             const key = value.toLowerCase();
-                            if (ts.hasProperty(optType, key)) {
+                            if (key in optType) {
                                 value = optType[key];
                             }
                         }
diff --git a/src/harness/unittests/cachingInServerLSHost.ts b/src/harness/unittests/cachingInServerLSHost.ts
index e0ce09391fb..2885b310605 100644
--- a/src/harness/unittests/cachingInServerLSHost.ts
+++ b/src/harness/unittests/cachingInServerLSHost.ts
@@ -7,16 +7,16 @@ namespace ts {
     }
 
     function createDefaultServerHost(fileMap: Map<File>): server.ServerHost {
-        const existingDirectories: Map<boolean> = {};
-        forEachValue(fileMap, v => {
-            let dir = getDirectoryPath(v.name);
+        const existingDirectories = createMap<boolean>();
+        for (const name in fileMap) {
+            let dir = getDirectoryPath(name);
             let previous: string;
             do {
                 existingDirectories[dir] = true;
                 previous = dir;
                 dir = getDirectoryPath(dir);
             } while (dir !== previous);
-        });
+        }
         return {
             args: <string[]>[],
             newLine: "\r\n",
@@ -24,7 +24,7 @@ namespace ts {
             write: (s: string) => {
             },
             readFile: (path: string, encoding?: string): string => {
-                return hasProperty(fileMap, path) && fileMap[path].content;
+                return path in fileMap ? fileMap[path].content : undefined;
             },
             writeFile: (path: string, data: string, writeByteOrderMark?: boolean) => {
                 throw new Error("NYI");
@@ -33,10 +33,10 @@ namespace ts {
                 throw new Error("NYI");
             },
             fileExists: (path: string): boolean => {
-                return hasProperty(fileMap, path);
+                return path in fileMap;
             },
             directoryExists: (path: string): boolean => {
-                return hasProperty(existingDirectories, path);
+                return existingDirectories[path] || false;
             },
             createDirectory: (path: string) => {
             },
@@ -101,7 +101,7 @@ namespace ts {
                 content: `foo()`
             };
 
-            const serverHost = createDefaultServerHost({ [root.name]: root, [imported.name]: imported });
+            const serverHost = createDefaultServerHost(createMap({ [root.name]: root, [imported.name]: imported }));
             const { project, rootScriptInfo } = createProject(root.name, serverHost);
 
             // ensure that imported file was found
@@ -193,7 +193,7 @@ namespace ts {
                 content: `export var y = 1`
             };
 
-            const fileMap: Map<File> = { [root.name]: root };
+            const fileMap = createMap({ [root.name]: root });
             const serverHost = createDefaultServerHost(fileMap);
             const originalFileExists = serverHost.fileExists;
 
diff --git a/src/harness/unittests/moduleResolution.ts b/src/harness/unittests/moduleResolution.ts
index 5f63889b084..09febd2300a 100644
--- a/src/harness/unittests/moduleResolution.ts
+++ b/src/harness/unittests/moduleResolution.ts
@@ -10,7 +10,7 @@ namespace ts {
         const map = arrayToMap(files, f => f.name);
 
         if (hasDirectoryExists) {
-            const directories: Map<string> = {};
+            const directories = createMap<string>();
             for (const f of files) {
                 let name = getDirectoryPath(f.name);
                 while (true) {
@@ -25,19 +25,19 @@ namespace ts {
             return {
                 readFile,
                 directoryExists: path => {
-                    return hasProperty(directories, path);
+                    return path in directories;
                 },
                 fileExists: path => {
-                    assert.isTrue(hasProperty(directories, getDirectoryPath(path)), `'fileExists' '${path}' request in non-existing directory`);
-                    return hasProperty(map, path);
+                    assert.isTrue(getDirectoryPath(path) in directories, `'fileExists' '${path}' request in non-existing directory`);
+                    return path in map;
                 }
             };
         }
         else {
-            return { readFile, fileExists: path => hasProperty(map, path), };
+            return { readFile, fileExists: path => path in map, };
         }
         function readFile(path: string): string {
-            return hasProperty(map, path) ? map[path].content : undefined;
+            return path in map ? map[path].content : undefined;
         }
     }
 
@@ -287,7 +287,7 @@ namespace ts {
             const host: CompilerHost = {
                 getSourceFile: (fileName: string, languageVersion: ScriptTarget) => {
                     const path = normalizePath(combinePaths(currentDirectory, fileName));
-                    return hasProperty(files, path) ? createSourceFile(fileName, files[path], languageVersion) : undefined;
+                    return path in files ? createSourceFile(fileName, files[path], languageVersion) : undefined;
                 },
                 getDefaultLibFileName: () => "lib.d.ts",
                 writeFile: (fileName, content): void => { throw new Error("NotImplemented"); },
@@ -298,7 +298,7 @@ namespace ts {
                 useCaseSensitiveFileNames: () => false,
                 fileExists: fileName => {
                     const path = normalizePath(combinePaths(currentDirectory, fileName));
-                    return hasProperty(files, path);
+                    return path in files;
                 },
                 readFile: (fileName): string => { throw new Error("NotImplemented"); }
             };
@@ -318,7 +318,7 @@ namespace ts {
         }
 
         it("should find all modules", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c/first/shared.ts": `
 class A {}
 export = A`,
@@ -332,23 +332,23 @@ import Shared = require('../first/shared');
 class C {}
 export = C;
                 `
-            };
+            });
             test(files, "/a/b/c/first/second", ["class_a.ts"], 3, ["../../../c/third/class_c.ts"]);
         });
 
         it("should find modules in node_modules", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/parent/node_modules/mod/index.d.ts": "export var x",
                 "/parent/app/myapp.ts": `import {x} from "mod"`
-            };
+            });
             test(files, "/parent/app", ["myapp.ts"], 2, []);
         });
 
         it("should find file referenced via absolute and relative names", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c.ts": `/// <reference path="b.ts"/>`,
                 "/a/b/b.ts": "var x"
-            };
+            });
             test(files, "/a/b", ["c.ts", "/a/b/b.ts"], 2, []);
         });
     });
@@ -358,11 +358,7 @@ export = C;
         function test(files: Map<string>, options: CompilerOptions, currentDirectory: string, useCaseSensitiveFileNames: boolean, rootFiles: string[], diagnosticCodes: number[]): void {
             const getCanonicalFileName = createGetCanonicalFileName(useCaseSensitiveFileNames);
             if (!useCaseSensitiveFileNames) {
-                const f: Map<string> = {};
-                for (const fileName in files) {
-                    f[getCanonicalFileName(fileName)] = files[fileName];
-                }
-                files = f;
+                files = reduceProperties(files, (files, file, fileName) => (files[getCanonicalFileName(fileName)] = file, files), createMap<string>());
             }
 
             const host: CompilerHost = {
@@ -371,7 +367,7 @@ export = C;
                         return library;
                     }
                     const path = getCanonicalFileName(normalizePath(combinePaths(currentDirectory, fileName)));
-                    return hasProperty(files, path) ? createSourceFile(fileName, files[path], languageVersion) : undefined;
+                    return path in files ? createSourceFile(fileName, files[path], languageVersion) : undefined;
                 },
                 getDefaultLibFileName: () => "lib.d.ts",
                 writeFile: (fileName, content): void => { throw new Error("NotImplemented"); },
@@ -382,7 +378,7 @@ export = C;
                 useCaseSensitiveFileNames: () => useCaseSensitiveFileNames,
                 fileExists: fileName => {
                     const path = getCanonicalFileName(normalizePath(combinePaths(currentDirectory, fileName)));
-                    return hasProperty(files, path);
+                    return path in files;
                 },
                 readFile: (fileName): string => { throw new Error("NotImplemented"); }
             };
@@ -395,57 +391,57 @@ export = C;
         }
 
         it("should succeed when the same file is referenced using absolute and relative names", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c.ts": `/// <reference path="d.ts"/>`,
                 "/a/b/d.ts": "var x"
-            };
+            });
             test(files, { module: ts.ModuleKind.AMD },  "/a/b", /*useCaseSensitiveFileNames*/ false, ["c.ts", "/a/b/d.ts"], []);
         });
 
         it("should fail when two files used in program differ only in casing (tripleslash references)", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c.ts": `/// <reference path="D.ts"/>`,
                 "/a/b/d.ts": "var x"
-            };
+            });
             test(files, { module: ts.ModuleKind.AMD, forceConsistentCasingInFileNames: true },  "/a/b", /*useCaseSensitiveFileNames*/ false, ["c.ts", "d.ts"], [1149]);
         });
 
         it("should fail when two files used in program differ only in casing (imports)", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c.ts": `import {x} from "D"`,
                 "/a/b/d.ts": "export var x"
-            };
+            });
             test(files, { module: ts.ModuleKind.AMD, forceConsistentCasingInFileNames: true },  "/a/b", /*useCaseSensitiveFileNames*/ false, ["c.ts", "d.ts"], [1149]);
         });
 
         it("should fail when two files used in program differ only in casing (imports, relative module names)", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "moduleA.ts": `import {x} from "./ModuleB"`,
                 "moduleB.ts": "export var x"
-            };
+            });
             test(files, { module: ts.ModuleKind.CommonJS, forceConsistentCasingInFileNames: true },  "", /*useCaseSensitiveFileNames*/ false, ["moduleA.ts", "moduleB.ts"], [1149]);
         });
 
         it("should fail when two files exist on disk that differs only in casing", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/b/c.ts": `import {x} from "D"`,
                 "/a/b/D.ts": "export var x",
                 "/a/b/d.ts": "export var y"
-            };
+            });
             test(files, { module: ts.ModuleKind.AMD },  "/a/b", /*useCaseSensitiveFileNames*/ true, ["c.ts", "d.ts"], [1149]);
         });
 
         it("should fail when module name in 'require' calls has inconsistent casing", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "moduleA.ts": `import a = require("./ModuleC")`,
                 "moduleB.ts": `import a = require("./moduleC")`,
                 "moduleC.ts": "export var x"
-            };
+            });
             test(files, { module: ts.ModuleKind.CommonJS, forceConsistentCasingInFileNames: true },  "", /*useCaseSensitiveFileNames*/ false, ["moduleA.ts", "moduleB.ts", "moduleC.ts"], [1149, 1149]);
         });
 
         it("should fail when module names in 'require' calls has inconsistent casing and current directory has uppercase chars", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/B/c/moduleA.ts": `import a = require("./ModuleC")`,
                 "/a/B/c/moduleB.ts": `import a = require("./moduleC")`,
                 "/a/B/c/moduleC.ts": "export var x",
@@ -453,11 +449,11 @@ export = C;
 import a = require("./moduleA.ts");
 import b = require("./moduleB.ts");
                 `
-            };
+            });
             test(files, { module: ts.ModuleKind.CommonJS, forceConsistentCasingInFileNames: true },  "/a/B/c", /*useCaseSensitiveFileNames*/ false, ["moduleD.ts"], [1149]);
         });
         it("should not fail when module names in 'require' calls has consistent casing and current directory has uppercase chars", () => {
-            const files: Map<string> = {
+            const files = createMap({
                 "/a/B/c/moduleA.ts": `import a = require("./moduleC")`,
                 "/a/B/c/moduleB.ts": `import a = require("./moduleC")`,
                 "/a/B/c/moduleC.ts": "export var x",
@@ -465,7 +461,7 @@ import b = require("./moduleB.ts");
 import a = require("./moduleA.ts");
 import b = require("./moduleB.ts");
                 `
-            };
+            });
             test(files, { module: ts.ModuleKind.CommonJS, forceConsistentCasingInFileNames: true },  "/a/B/c", /*useCaseSensitiveFileNames*/ false, ["moduleD.ts"], []);
         });
     });
@@ -1023,7 +1019,7 @@ import b = require("./moduleB.ts");
             const names = map(files, f => f.name);
             const sourceFiles = arrayToMap(map(files, f => createSourceFile(f.name, f.content, ScriptTarget.ES6)), f => f.fileName);
             const compilerHost: CompilerHost = {
-                fileExists : fileName => hasProperty(sourceFiles, fileName),
+                fileExists : fileName => fileName in sourceFiles,
                 getSourceFile: fileName => sourceFiles[fileName],
                 getDefaultLibFileName: () => "lib.d.ts",
                 writeFile(file, text) {
@@ -1034,7 +1030,7 @@ import b = require("./moduleB.ts");
                 getCanonicalFileName: f => f.toLowerCase(),
                 getNewLine: () => "\r\n",
                 useCaseSensitiveFileNames: () => false,
-                readFile: fileName => hasProperty(sourceFiles, fileName) ? sourceFiles[fileName].text : undefined
+                readFile: fileName => fileName in sourceFiles ? sourceFiles[fileName].text : undefined
             };
             const program1 = createProgram(names, {}, compilerHost);
             const diagnostics1 = program1.getFileProcessingDiagnostics().getDiagnostics();
diff --git a/src/harness/unittests/reuseProgramStructure.ts b/src/harness/unittests/reuseProgramStructure.ts
index 8b2b15c15b3..60687d34c55 100644
--- a/src/harness/unittests/reuseProgramStructure.ts
+++ b/src/harness/unittests/reuseProgramStructure.ts
@@ -95,13 +95,14 @@ namespace ts {
         }
     }
 
+    function createSourceFileWithText(fileName: string, sourceText: SourceText, target: ScriptTarget) {
+        const file = <SourceFileWithText>createSourceFile(fileName, sourceText.getFullText(), target);
+        file.sourceText = sourceText;
+        return file;
+    }
+
     function createTestCompilerHost(texts: NamedSourceText[], target: ScriptTarget): CompilerHost {
-        const files: Map<SourceFileWithText> = {};
-        for (const t of texts) {
-            const file = <SourceFileWithText>createSourceFile(t.name, t.text.getFullText(), target);
-            file.sourceText = t.text;
-            files[t.name] = file;
-        }
+        const files = arrayToMap(texts, t => t.name, t => createSourceFileWithText(t.name, t.text, target));
 
         return {
             getSourceFile(fileName): SourceFile {
@@ -128,10 +129,9 @@ namespace ts {
             getNewLine(): string {
                 return sys ? sys.newLine : newLine;
             },
-            fileExists: fileName => hasProperty(files, fileName),
+            fileExists: fileName => fileName in files,
             readFile: fileName => {
-                const file = lookUp(files, fileName);
-                return file && file.text;
+                return fileName in files ? files[fileName].text : undefined;
             }
         };
     }
@@ -152,29 +152,29 @@ namespace ts {
         return program;
     }
 
-    function getSizeOfMap(map: Map<any>): number {
-        let size = 0;
-        for (const id in map) {
-            if (hasProperty(map, id)) {
-                size++;
+    function checkResolvedModule(expected: ResolvedModule, actual: ResolvedModule): boolean {
+        if (!expected === !actual) {
+            if (expected) {
+                assert.isTrue(expected.resolvedFileName === actual.resolvedFileName, `'resolvedFileName': expected '${expected.resolvedFileName}' to be equal to '${actual.resolvedFileName}'`);
+                assert.isTrue(expected.isExternalLibraryImport === actual.isExternalLibraryImport, `'isExternalLibraryImport': expected '${expected.isExternalLibraryImport}' to be equal to '${actual.isExternalLibraryImport}'`);
             }
+            return true;
         }
-        return size;
+        return false;
     }
 
-    function checkResolvedModule(expected: ResolvedModule, actual: ResolvedModule): void {
-        assert.isTrue(actual !== undefined);
-        assert.isTrue(expected.resolvedFileName === actual.resolvedFileName, `'resolvedFileName': expected '${expected.resolvedFileName}' to be equal to '${actual.resolvedFileName}'`);
-        assert.isTrue(expected.isExternalLibraryImport === actual.isExternalLibraryImport, `'isExternalLibraryImport': expected '${expected.isExternalLibraryImport}' to be equal to '${actual.isExternalLibraryImport}'`);
+    function checkResolvedTypeDirective(expected: ResolvedTypeReferenceDirective, actual: ResolvedTypeReferenceDirective): boolean {
+        if (!expected === !actual) {
+            if (expected) {
+                assert.isTrue(expected.resolvedFileName === actual.resolvedFileName, `'resolvedFileName': expected '${expected.resolvedFileName}' to be equal to '${actual.resolvedFileName}'`);
+                assert.isTrue(expected.primary === actual.primary, `'primary': expected '${expected.primary}' to be equal to '${actual.primary}'`);
+            }
+            return true;
+        }
+        return false;
     }
 
-    function checkResolvedTypeDirective(expected: ResolvedTypeReferenceDirective, actual: ResolvedTypeReferenceDirective): void {
-        assert.isTrue(actual !== undefined);
-        assert.isTrue(expected.resolvedFileName === actual.resolvedFileName, `'resolvedFileName': expected '${expected.resolvedFileName}' to be equal to '${actual.resolvedFileName}'`);
-        assert.isTrue(expected.primary === actual.primary, `'primary': expected '${expected.primary}' to be equal to '${actual.primary}'`);
-    }
-
-    function checkCache<T>(caption: string, program: Program, fileName: string, expectedContent: Map<T>, getCache: (f: SourceFile) => Map<T>, entryChecker: (expected: T, original: T) => void): void {
+    function checkCache<T>(caption: string, program: Program, fileName: string, expectedContent: Map<T>, getCache: (f: SourceFile) => Map<T>, entryChecker: (expected: T, original: T) => boolean): void {
         const file = program.getSourceFile(fileName);
         assert.isTrue(file !== undefined, `cannot find file ${fileName}`);
         const cache = getCache(file);
@@ -183,23 +183,7 @@ namespace ts {
         }
         else {
             assert.isTrue(cache !== undefined, `expected ${caption} to be set`);
-            const actualCacheSize = getSizeOfMap(cache);
-            const expectedSize = getSizeOfMap(expectedContent);
-            assert.isTrue(actualCacheSize === expectedSize, `expected actual size: ${actualCacheSize} to be equal to ${expectedSize}`);
-
-            for (const id in expectedContent) {
-                if (hasProperty(expectedContent, id)) {
-
-                    if (expectedContent[id]) {
-                        const expected = expectedContent[id];
-                        const actual = cache[id];
-                        entryChecker(expected, actual);
-                    }
-                }
-                else {
-                    assert.isTrue(cache[id] === undefined);
-                }
-            }
+            assert.isTrue(equalOwnProperties(expectedContent, cache, entryChecker), `contents of ${caption} did not match the expected contents.`);
         }
     }
 
@@ -313,6 +297,8 @@ namespace ts {
         });
 
         it("resolution cache follows imports", () => {
+            (<any>Error).stackTraceLimit = Infinity;
+
             const files = [
                 { name: "a.ts", text: SourceText.New("", "import {_} from 'b'", "var x = 1") },
                 { name: "b.ts", text: SourceText.New("", "", "var y = 2") },
@@ -320,7 +306,7 @@ namespace ts {
             const options: CompilerOptions = { target };
 
             const program_1 = newProgram(files, ["a.ts"], options);
-            checkResolvedModulesCache(program_1, "a.ts", { "b": { resolvedFileName: "b.ts" } });
+            checkResolvedModulesCache(program_1, "a.ts", createMap({ "b": { resolvedFileName: "b.ts" } }));
             checkResolvedModulesCache(program_1, "b.ts", undefined);
 
             const program_2 = updateProgram(program_1, ["a.ts"], options, files => {
@@ -329,7 +315,7 @@ namespace ts {
             assert.isTrue(program_1.structureIsReused);
 
             // content of resolution cache should not change
-            checkResolvedModulesCache(program_1, "a.ts", { "b": { resolvedFileName: "b.ts" } });
+            checkResolvedModulesCache(program_1, "a.ts", createMap({ "b": { resolvedFileName: "b.ts" } }));
             checkResolvedModulesCache(program_1, "b.ts", undefined);
 
             // imports has changed - program is not reused
@@ -346,7 +332,7 @@ namespace ts {
                 files[0].text = files[0].text.updateImportsAndExports(newImports);
             });
             assert.isTrue(!program_3.structureIsReused);
-            checkResolvedModulesCache(program_4, "a.ts", { "b": { resolvedFileName: "b.ts" }, "c": undefined });
+            checkResolvedModulesCache(program_4, "a.ts", createMap({ "b": { resolvedFileName: "b.ts" }, "c": undefined }));
         });
 
         it("resolved type directives cache follows type directives", () => {
@@ -357,7 +343,7 @@ namespace ts {
             const options: CompilerOptions = { target, typeRoots: ["/types"] };
 
             const program_1 = newProgram(files, ["/a.ts"], options);
-            checkResolvedTypeDirectivesCache(program_1, "/a.ts", { "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } });
+            checkResolvedTypeDirectivesCache(program_1, "/a.ts", createMap({ "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } }));
             checkResolvedTypeDirectivesCache(program_1, "/types/typedefs/index.d.ts", undefined);
 
             const program_2 = updateProgram(program_1, ["/a.ts"], options, files => {
@@ -366,7 +352,7 @@ namespace ts {
             assert.isTrue(program_1.structureIsReused);
 
             // content of resolution cache should not change
-            checkResolvedTypeDirectivesCache(program_1, "/a.ts", { "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } });
+            checkResolvedTypeDirectivesCache(program_1, "/a.ts", createMap({ "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } }));
             checkResolvedTypeDirectivesCache(program_1, "/types/typedefs/index.d.ts", undefined);
 
             // type reference directives has changed - program is not reused
@@ -384,7 +370,7 @@ namespace ts {
                 files[0].text = files[0].text.updateReferences(newReferences);
             });
             assert.isTrue(!program_3.structureIsReused);
-            checkResolvedTypeDirectivesCache(program_1, "/a.ts", { "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } });
+            checkResolvedTypeDirectivesCache(program_1, "/a.ts", createMap({ "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } }));
         });
     });
 
diff --git a/src/harness/unittests/session.ts b/src/harness/unittests/session.ts
index c5285544329..cf7486acb40 100644
--- a/src/harness/unittests/session.ts
+++ b/src/harness/unittests/session.ts
@@ -106,7 +106,7 @@ namespace ts.server {
         describe("onMessage", () => {
             it("should not throw when commands are executed with invalid arguments", () => {
                 let i = 0;
-                for (name in CommandNames) {
+                for (const name in CommandNames) {
                     if (!Object.prototype.hasOwnProperty.call(CommandNames, name)) {
                         continue;
                     }
@@ -362,13 +362,13 @@ namespace ts.server {
         class InProcClient {
             private server: InProcSession;
             private seq = 0;
-            private callbacks: ts.Map<(resp: protocol.Response) => void> = {};
-            private eventHandlers: ts.Map<(args: any) => void> = {};
+            private callbacks = createMap<(resp: protocol.Response) => void>();
+            private eventHandlers = createMap<(args: any) => void>();
 
             handle(msg: protocol.Message): void {
                 if (msg.type === "response") {
                     const response = <protocol.Response>msg;
-                    if (this.callbacks[response.request_seq]) {
+                    if (response.request_seq in this.callbacks) {
                         this.callbacks[response.request_seq](response);
                         delete this.callbacks[response.request_seq];
                     }
@@ -380,7 +380,7 @@ namespace ts.server {
             }
 
             emit(name: string, args: any): void {
-                if (this.eventHandlers[name]) {
+                if (name in this.eventHandlers) {
                     this.eventHandlers[name](args);
                 }
             }
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index 7a375c50b8a..49f033b9def 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -68,20 +68,10 @@ namespace ts {
         return entry;
     }
 
-    function sizeOfMap(map: Map<any>): number {
-        let n = 0;
-        for (const name in map) {
-            if (hasProperty(map, name)) {
-                n++;
-            }
-        }
-        return n;
-    }
-
     function checkMapKeys(caption: string, map: Map<any>, expectedKeys: string[]) {
-        assert.equal(sizeOfMap(map), expectedKeys.length, `${caption}: incorrect size of map`);
+        assert.equal(reduceProperties(map, count => count + 1, 0), expectedKeys.length, `${caption}: incorrect size of map`);
         for (const name of expectedKeys) {
-            assert.isTrue(hasProperty(map, name), `${caption} is expected to contain ${name}, actual keys: ${getKeys(map)}`);
+            assert.isTrue(name in map, `${caption} is expected to contain ${name}, actual keys: ${Object.keys(map)}`);
         }
     }
 
@@ -126,8 +116,8 @@ namespace ts {
         private getCanonicalFileName: (s: string) => string;
         private toPath: (f: string) => Path;
         private callbackQueue: TimeOutCallback[] = [];
-        readonly watchedDirectories: Map<{ cb: DirectoryWatcherCallback, recursive: boolean }[]> = {};
-        readonly watchedFiles: Map<FileWatcherCallback[]> = {};
+        readonly watchedDirectories = createMap<{ cb: DirectoryWatcherCallback, recursive: boolean }[]>();
+        readonly watchedFiles = createMap<FileWatcherCallback[]>();
 
         constructor(public useCaseSensitiveFileNames: boolean, private executingFilePath: string, private currentDirectory: string, fileOrFolderList: FileOrFolder[]) {
             this.getCanonicalFileName = createGetCanonicalFileName(useCaseSensitiveFileNames);
@@ -208,7 +198,7 @@ namespace ts {
 
         watchDirectory(directoryName: string, callback: DirectoryWatcherCallback, recursive: boolean): DirectoryWatcher {
             const path = this.toPath(directoryName);
-            const callbacks = lookUp(this.watchedDirectories, path) || (this.watchedDirectories[path] = []);
+            const callbacks = this.watchedDirectories[path] || (this.watchedDirectories[path] = []);
             callbacks.push({ cb: callback, recursive });
             return {
                 referenceCount: 0,
@@ -229,7 +219,7 @@ namespace ts {
 
         triggerDirectoryWatcherCallback(directoryName: string, fileName: string): void {
             const path = this.toPath(directoryName);
-            const callbacks = lookUp(this.watchedDirectories, path);
+            const callbacks = this.watchedDirectories[path];
             if (callbacks) {
                 for (const callback of callbacks) {
                     callback.cb(fileName);
@@ -239,7 +229,7 @@ namespace ts {
 
         triggerFileWatcherCallback(fileName: string, removed?: boolean): void {
             const path = this.toPath(fileName);
-            const callbacks = lookUp(this.watchedFiles, path);
+            const callbacks = this.watchedFiles[path];
             if (callbacks) {
                 for (const callback of callbacks) {
                     callback(path, removed);
@@ -249,7 +239,7 @@ namespace ts {
 
         watchFile(fileName: string, callback: FileWatcherCallback) {
             const path = this.toPath(fileName);
-            const callbacks = lookUp(this.watchedFiles, path) || (this.watchedFiles[path] = []);
+            const callbacks = this.watchedFiles[path] || (this.watchedFiles[path] = []);
             callbacks.push(callback);
             return {
                 close: () => {
@@ -594,7 +584,7 @@ namespace ts {
                 content: `{
                     "compilerOptions": {
                         "target": "es6"
-                    }, 
+                    },
                     "files": [ "main.ts" ]
                 }`
             };
@@ -621,7 +611,7 @@ namespace ts {
                 content: `{
                     "compilerOptions": {
                         "target": "es6"
-                    }, 
+                    },
                     "files": [ "main.ts" ]
                 }`
             };
diff --git a/src/lib/es2015.core.d.ts b/src/lib/es2015.core.d.ts
index fef0b0ce3ce..15fbb1b77c5 100644
--- a/src/lib/es2015.core.d.ts
+++ b/src/lib/es2015.core.d.ts
@@ -68,6 +68,10 @@ interface ArrayConstructor {
     of<T>(...items: T[]): Array<T>;
 }
 
+interface DateConstructor {
+    new (value: Date): Date;
+}
+
 interface Function {
     /**
       * Returns the name of the function. Function names are read-only and can not be changed.
@@ -343,6 +347,30 @@ interface ObjectConstructor {
     defineProperty(o: any, propertyKey: PropertyKey, attributes: PropertyDescriptor): any;
 }
 
+interface ReadonlyArray<T> {
+  /**
+    * Returns the value of the first element in the array where predicate is true, and undefined
+    * otherwise.
+    * @param predicate find calls predicate once for each element of the array, in ascending
+    * order, until it finds one where predicate returns true. If such an element is found, find
+    * immediately returns that element value. Otherwise, find returns undefined.
+    * @param thisArg If provided, it will be used as the this value for each invocation of
+    * predicate. If it is not provided, undefined is used instead.
+    */
+  find(predicate: (value: T, index: number, obj: ReadonlyArray<T>) => boolean, thisArg?: any): T | undefined;
+
+  /**
+    * Returns the index of the first element in the array where predicate is true, and undefined
+    * otherwise.
+    * @param predicate find calls predicate once for each element of the array, in ascending
+    * order, until it finds one where predicate returns true. If such an element is found, 
+    * findIndex immediately returns that element index. Otherwise, findIndex returns -1.
+    * @param thisArg If provided, it will be used as the this value for each invocation of
+    * predicate. If it is not provided, undefined is used instead.
+    */
+  findIndex(predicate: (value: T) => boolean, thisArg?: any): number;
+}
+
 interface RegExp {
     /**
       * Returns a string indicating the flags of the regular expression in question. This field is read-only.
diff --git a/src/lib/es2015.iterable.d.ts b/src/lib/es2015.iterable.d.ts
index e1b9d99762b..de339e2bf2c 100644
--- a/src/lib/es2015.iterable.d.ts
+++ b/src/lib/es2015.iterable.d.ts
@@ -63,6 +63,26 @@ interface ArrayConstructor {
     from<T>(iterable: Iterable<T>): Array<T>;
 }
 
+interface ReadonlyArray<T> {
+    /** Iterator */
+    [Symbol.iterator](): IterableIterator<T>;
+
+    /** 
+      * Returns an array of key, value pairs for every entry in the array
+      */
+    entries(): IterableIterator<[number, T]>;
+
+    /** 
+      * Returns an list of keys in the array
+      */
+    keys(): IterableIterator<number>;
+
+    /** 
+      * Returns an list of values in the array
+      */
+    values(): IterableIterator<T>;
+}
+
 interface IArguments {
     /** Iterator */
     [Symbol.iterator](): IterableIterator<any>;
diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 496df578c19..e4cbe323c68 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1006,7 +1006,12 @@ interface ReadonlyArray<T> {
       * Combines two or more arrays.
       * @param items Additional items to add to the end of array1.
       */
-    concat(...items: T[]): T[];
+    concat(...items: T[][]): T[];
+    /**
+      * Combines two or more arrays.
+      * @param items Additional items to add to the end of array1.
+      */
+    concat(...items: (T | T[])[]): T[];
     /**
       * Adds all the elements of an array separated by the specified separator string.
       * @param separator A string used to separate one element of an array from the next in the resulting String. If omitted, the array elements are separated with a comma.
diff --git a/src/server/client.ts b/src/server/client.ts
index f04dbd8dc02..88177e91fad 100644
--- a/src/server/client.ts
+++ b/src/server/client.ts
@@ -21,7 +21,7 @@ namespace ts.server {
 
     export class SessionClient implements LanguageService {
         private sequence: number = 0;
-        private lineMaps: ts.Map<number[]> = {};
+        private lineMaps: ts.Map<number[]> = ts.createMap<number[]>();
         private messages: string[] = [];
         private lastRenameEntry: RenameEntry;
 
@@ -37,7 +37,7 @@ namespace ts.server {
         }
 
         private getLineMap(fileName: string): number[] {
-            let lineMap = ts.lookUp(this.lineMaps, fileName);
+            let lineMap = this.lineMaps[fileName];
             if (!lineMap) {
                 const scriptSnapshot = this.host.getScriptSnapshot(fileName);
                 lineMap = this.lineMaps[fileName] = ts.computeLineStarts(scriptSnapshot.getText(0, scriptSnapshot.getLength()));
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 6e9adad7f47..db9bdd5043b 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -130,15 +130,15 @@ namespace ts.server {
             const path = toPath(containingFile, this.host.getCurrentDirectory(), this.getCanonicalFileName);
             const currentResolutionsInFile = cache.get(path);
 
-            const newResolutions: Map<T> = {};
+            const newResolutions = createMap<T>();
             const resolvedModules: R[] = [];
             const compilerOptions = this.getCompilationSettings();
 
             for (const name of names) {
                 // check if this is a duplicate entry in the list
-                let resolution = lookUp(newResolutions, name);
+                let resolution = newResolutions[name];
                 if (!resolution) {
-                    const existingResolution = currentResolutionsInFile && ts.lookUp(currentResolutionsInFile, name);
+                    const existingResolution = currentResolutionsInFile && currentResolutionsInFile[name];
                     if (moduleResolutionIsValid(existingResolution)) {
                         // ok, it is safe to use existing name resolution results
                         resolution = existingResolution;
@@ -378,7 +378,7 @@ namespace ts.server {
     export interface ProjectOptions {
         // these fields can be present in the project file
         files?: string[];
-        wildcardDirectories?: ts.Map<ts.WatchDirectoryFlags>;
+        wildcardDirectories?: ts.MapLike<ts.WatchDirectoryFlags>;
         compilerOptions?: ts.CompilerOptions;
     }
 
@@ -391,7 +391,7 @@ namespace ts.server {
         // Used to keep track of what directories are watched for this project
         directoriesWatchedForTsconfig: string[] = [];
         program: ts.Program;
-        filenameToSourceFile: ts.Map<ts.SourceFile> = {};
+        filenameToSourceFile = ts.createMap<ts.SourceFile>();
         updateGraphSeq = 0;
         /** Used for configured projects which may have multiple open roots */
         openRefCount = 0;
@@ -504,7 +504,7 @@ namespace ts.server {
                 return;
             }
 
-            this.filenameToSourceFile = {};
+            this.filenameToSourceFile = createMap<SourceFile>();
             const sourceFiles = this.program.getSourceFiles();
             for (let i = 0, len = sourceFiles.length; i < len; i++) {
                 const normFilename = ts.normalizePath(sourceFiles[i].fileName);
@@ -563,7 +563,7 @@ namespace ts.server {
             }
 
             let strBuilder = "";
-            ts.forEachValue(this.filenameToSourceFile,
+            ts.forEachProperty(this.filenameToSourceFile,
                 sourceFile => { strBuilder += sourceFile.fileName + "\n"; });
             return strBuilder;
         }
@@ -613,7 +613,7 @@ namespace ts.server {
     }
 
     export class ProjectService {
-        filenameToScriptInfo: ts.Map<ScriptInfo> = {};
+        filenameToScriptInfo = ts.createMap<ScriptInfo>();
         // open, non-configured root files
         openFileRoots: ScriptInfo[] = [];
         // projects built from openFileRoots
@@ -625,12 +625,12 @@ namespace ts.server {
         // open files that are roots of a configured project
         openFileRootsConfigured: ScriptInfo[] = [];
         // a path to directory watcher map that detects added tsconfig files
-        directoryWatchersForTsconfig: ts.Map<FileWatcher> = {};
+        directoryWatchersForTsconfig = ts.createMap<FileWatcher>();
         // count of how many projects are using the directory watcher. If the
         // number becomes 0 for a watcher, then we should close it.
-        directoryWatchersRefCount: ts.Map<number> = {};
+        directoryWatchersRefCount = ts.createMap<number>();
         hostConfiguration: HostConfiguration;
-        timerForDetectingProjectFileListChanges: Map<any> = {};
+        timerForDetectingProjectFileListChanges = createMap<any>();
 
         constructor(public host: ServerHost, public psLogger: Logger, public eventHandler?: ProjectServiceEventHandler) {
             // ts.disableIncrementalParsing = true;
@@ -857,7 +857,7 @@ namespace ts.server {
             if (project.isConfiguredProject()) {
                 project.projectFileWatcher.close();
                 project.directoryWatcher.close();
-                forEachValue(project.directoriesWatchedForWildcards, watcher => { watcher.close(); });
+                forEachProperty(project.directoriesWatchedForWildcards, watcher => { watcher.close(); });
                 delete project.directoriesWatchedForWildcards;
                 this.configuredProjects = copyListRemovingItem(project, this.configuredProjects);
             }
@@ -1124,7 +1124,7 @@ namespace ts.server {
 
         getScriptInfo(filename: string) {
             filename = ts.normalizePath(filename);
-            return ts.lookUp(this.filenameToScriptInfo, filename);
+            return this.filenameToScriptInfo[filename];
         }
 
         /**
@@ -1133,7 +1133,7 @@ namespace ts.server {
          */
         openFile(fileName: string, openedByClient: boolean, fileContent?: string, scriptKind?: ScriptKind) {
             fileName = ts.normalizePath(fileName);
-            let info = ts.lookUp(this.filenameToScriptInfo, fileName);
+            let info = this.filenameToScriptInfo[fileName];
             if (!info) {
                 let content: string;
                 if (this.host.fileExists(fileName)) {
@@ -1246,7 +1246,7 @@ namespace ts.server {
          * @param filename is absolute pathname
          */
         closeClientFile(filename: string) {
-            const info = ts.lookUp(this.filenameToScriptInfo, filename);
+            const info = this.filenameToScriptInfo[filename];
             if (info) {
                 this.closeOpenFile(info);
                 info.isOpen = false;
@@ -1255,14 +1255,14 @@ namespace ts.server {
         }
 
         getProjectForFile(filename: string) {
-            const scriptInfo = ts.lookUp(this.filenameToScriptInfo, filename);
+            const scriptInfo = this.filenameToScriptInfo[filename];
             if (scriptInfo) {
                 return scriptInfo.defaultProject;
             }
         }
 
         printProjectsForFile(filename: string) {
-            const scriptInfo = ts.lookUp(this.filenameToScriptInfo, filename);
+            const scriptInfo = this.filenameToScriptInfo[filename];
             if (scriptInfo) {
                 this.psLogger.startGroup();
                 this.psLogger.info("Projects for " + filename);
@@ -1419,7 +1419,7 @@ namespace ts.server {
                     /*recursive*/ true
                 );
 
-                project.directoriesWatchedForWildcards = reduceProperties(projectOptions.wildcardDirectories, (watchers, flag, directory) => {
+                project.directoriesWatchedForWildcards = reduceProperties(createMap(projectOptions.wildcardDirectories), (watchers, flag, directory) => {
                     if (comparePaths(configDirectoryPath, directory, ".", !this.host.useCaseSensitiveFileNames) !== Comparison.EqualTo) {
                         const recursive = (flag & WatchDirectoryFlags.Recursive) !== 0;
                         this.log(`Add ${ recursive ? "recursive " : ""}watcher for: ${directory}`);
diff --git a/src/server/session.ts b/src/server/session.ts
index 7e1ca81e2af..9383a54bd48 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -1061,7 +1061,7 @@ namespace ts.server {
             return { response, responseRequired: true };
         }
 
-        private handlers: Map<(request: protocol.Request) => { response?: any, responseRequired?: boolean }> = {
+        private handlers = createMap<(request: protocol.Request) => { response?: any, responseRequired?: boolean }>({
             [CommandNames.Exit]: () => {
                 this.exit();
                 return { responseRequired: false };
@@ -1198,9 +1198,10 @@ namespace ts.server {
                 this.reloadProjects();
                 return { responseRequired: false };
             }
-        };
+        });
+
         public addProtocolHandler(command: string, handler: (request: protocol.Request) => { response?: any, responseRequired: boolean }) {
-            if (this.handlers[command]) {
+            if (command in this.handlers) {
                 throw new Error(`Protocol handler already exists for command "${command}"`);
             }
             this.handlers[command] = handler;
diff --git a/src/server/tsconfig.library.json b/src/server/tsconfig.library.json
index 746283720bf..af04a74d557 100644
--- a/src/server/tsconfig.library.json
+++ b/src/server/tsconfig.library.json
@@ -10,6 +10,8 @@
         "types": []
     },
     "files": [
+        "../services/shims.ts",
+        "../services/utilities.ts",
         "editorServices.ts",
         "protocol.d.ts",
         "session.ts"
diff --git a/src/services/jsTyping.ts b/src/services/jsTyping.ts
index 3b013a4a924..4f0e06244d2 100644
--- a/src/services/jsTyping.ts
+++ b/src/services/jsTyping.ts
@@ -47,7 +47,7 @@ namespace ts.JsTyping {
         { cachedTypingPaths: string[], newTypingNames: string[], filesToWatch: string[] } {
 
         // A typing name to typing file path mapping
-        const inferredTypings: Map<string> = {};
+        const inferredTypings = createMap<string>();
 
         if (!typingOptions || !typingOptions.enableAutoDiscovery) {
             return { cachedTypingPaths: [], newTypingNames: [], filesToWatch: [] };
@@ -58,12 +58,7 @@ namespace ts.JsTyping {
 
         if (!safeList) {
             const result = readConfigFile(safeListPath, (path: string) => host.readFile(path));
-            if (result.config) {
-                safeList = result.config;
-            }
-            else {
-                safeList = {};
-            };
+            safeList = createMap<string>(result.config);
         }
 
         const filesToWatch: string[] = [];
@@ -93,7 +88,7 @@ namespace ts.JsTyping {
 
         // Add the cached typing locations for inferred typings that are already installed
         for (const name in packageNameToTypingLocation) {
-            if (hasProperty(inferredTypings, name) && !inferredTypings[name]) {
+            if (name in inferredTypings && !inferredTypings[name]) {
                 inferredTypings[name] = packageNameToTypingLocation[name];
             }
         }
@@ -124,7 +119,7 @@ namespace ts.JsTyping {
             }
 
             for (const typing of typingNames) {
-                if (!hasProperty(inferredTypings, typing)) {
+                if (!(typing in inferredTypings)) {
                     inferredTypings[typing] = undefined;
                 }
             }
@@ -139,16 +134,16 @@ namespace ts.JsTyping {
                 const jsonConfig: PackageJson = result.config;
                 filesToWatch.push(jsonPath);
                 if (jsonConfig.dependencies) {
-                    mergeTypings(getKeys(jsonConfig.dependencies));
+                    mergeTypings(getOwnKeys(jsonConfig.dependencies));
                 }
                 if (jsonConfig.devDependencies) {
-                    mergeTypings(getKeys(jsonConfig.devDependencies));
+                    mergeTypings(getOwnKeys(jsonConfig.devDependencies));
                 }
                 if (jsonConfig.optionalDependencies) {
-                    mergeTypings(getKeys(jsonConfig.optionalDependencies));
+                    mergeTypings(getOwnKeys(jsonConfig.optionalDependencies));
                 }
                 if (jsonConfig.peerDependencies) {
-                    mergeTypings(getKeys(jsonConfig.peerDependencies));
+                    mergeTypings(getOwnKeys(jsonConfig.peerDependencies));
                 }
             }
         }
@@ -167,7 +162,7 @@ namespace ts.JsTyping {
                 mergeTypings(cleanedTypingNames);
             }
             else {
-                mergeTypings(filter(cleanedTypingNames, f => hasProperty(safeList, f)));
+                mergeTypings(filter(cleanedTypingNames, f => f in safeList));
             }
 
             const hasJsxFile = forEach(fileNames, f => scriptKindIs(f, /*LanguageServiceHost*/ undefined, ScriptKind.JSX));
diff --git a/src/services/navigateTo.ts b/src/services/navigateTo.ts
index e9afb6925b5..ccded188e61 100644
--- a/src/services/navigateTo.ts
+++ b/src/services/navigateTo.ts
@@ -15,7 +15,7 @@ namespace ts.NavigateTo {
 
             const nameToDeclarations = sourceFile.getNamedDeclarations();
             for (const name in nameToDeclarations) {
-                const declarations = getProperty(nameToDeclarations, name);
+                const declarations = nameToDeclarations[name];
                 if (declarations) {
                     // First do a quick check to see if the name of the declaration matches the
                     // last portion of the (possibly) dotted name they're searching for.
diff --git a/src/services/navigationBar.ts b/src/services/navigationBar.ts
index f105a9fa6e2..dda9ef485d2 100644
--- a/src/services/navigationBar.ts
+++ b/src/services/navigationBar.ts
@@ -234,7 +234,7 @@ namespace ts.NavigationBar {
 
     /** Merge declarations of the same kind. */
     function mergeChildren(children: NavigationBarNode[]): void {
-        const nameToItems: Map<NavigationBarNode | NavigationBarNode[]> = {};
+        const nameToItems = createMap<NavigationBarNode | NavigationBarNode[]>();
         filterMutate(children, child => {
             const decl = <Declaration>child.node;
             const name = decl.name && nodeText(decl.name);
@@ -243,7 +243,7 @@ namespace ts.NavigationBar {
                 return true;
             }
 
-            const itemsWithSameName = getProperty(nameToItems, name);
+            const itemsWithSameName = nameToItems[name];
             if (!itemsWithSameName) {
                 nameToItems[name] = child;
                 return true;
diff --git a/src/services/patternMatcher.ts b/src/services/patternMatcher.ts
index 3d20337eca7..b4f67ca056d 100644
--- a/src/services/patternMatcher.ts
+++ b/src/services/patternMatcher.ts
@@ -113,7 +113,7 @@ namespace ts {
         // we see the name of a module that is used everywhere, or the name of an overload).  As
         // such, we cache the information we compute about the candidate for the life of this
         // pattern matcher so we don't have to compute it multiple times.
-        const stringToWordSpans: Map<TextSpan[]> = {};
+        const stringToWordSpans = createMap<TextSpan[]>();
 
         pattern = pattern.trim();
 
@@ -188,7 +188,7 @@ namespace ts {
         }
 
         function getWordSpans(word: string): TextSpan[] {
-            if (!hasProperty(stringToWordSpans, word)) {
+            if (!(word in stringToWordSpans)) {
                 stringToWordSpans[word] = breakIntoWordSpans(word);
             }
 
diff --git a/src/services/services.ts b/src/services/services.ts
index 6ec95343e33..148d3af427c 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -975,7 +975,7 @@ namespace ts {
         }
 
         private computeNamedDeclarations(): Map<Declaration[]> {
-            const result: Map<Declaration[]> = {};
+            const result = createMap<Declaration[]>();
 
             forEachChild(this, visit);
 
@@ -990,7 +990,7 @@ namespace ts {
             }
 
             function getDeclarations(name: string) {
-                return getProperty(result, name) || (result[name] = []);
+                return result[name] || (result[name] = []);
             }
 
             function getDeclarationName(declaration: Declaration) {
@@ -1886,7 +1886,7 @@ namespace ts {
         };
     }
 
-    // Cache host information about scrip Should be refreshed
+    // Cache host information about script should be refreshed
     // at each language service public entry point, since we don't know when
     // set of scripts handled by the host changes.
     class HostCache {
@@ -2025,7 +2025,7 @@ namespace ts {
         fileName?: string;
         reportDiagnostics?: boolean;
         moduleName?: string;
-        renamedDependencies?: Map<string>;
+        renamedDependencies?: MapLike<string>;
     }
 
     export interface TranspileOutput {
@@ -2042,7 +2042,7 @@ namespace ts {
     function fixupCompilerOptions(options: CompilerOptions, diagnostics: Diagnostic[]): CompilerOptions {
         // Lazily create this value to fix module loading errors.
         commandLineOptionsStringToEnum = commandLineOptionsStringToEnum || <CommandLineOptionOfCustomType[]>filter(optionDeclarations, o =>
-            typeof o.type === "object" && !forEachValue(<Map<any>>o.type, v => typeof v !== "number"));
+            typeof o.type === "object" && !forEachProperty(o.type, v => typeof v !== "number"));
 
         options = clone(options);
 
@@ -2058,7 +2058,7 @@ namespace ts {
                 options[opt.name] = parseCustomTypeOption(opt, value, diagnostics);
             }
             else {
-                if (!forEachValue(opt.type, v => v === value)) {
+                if (!forEachProperty(opt.type, v => v === value)) {
                     // Supplied value isn't a valid enum value.
                     diagnostics.push(createCompilerDiagnosticForInvalidCustomType(opt));
                 }
@@ -2117,7 +2117,9 @@ namespace ts {
             sourceFile.moduleName = transpileOptions.moduleName;
         }
 
-        sourceFile.renamedDependencies = transpileOptions.renamedDependencies;
+        if (transpileOptions.renamedDependencies) {
+            sourceFile.renamedDependencies = createMap(transpileOptions.renamedDependencies);
+        }
 
         const newLine = getNewLineCharacter(options);
 
@@ -2243,7 +2245,7 @@ namespace ts {
     export function createDocumentRegistry(useCaseSensitiveFileNames?: boolean, currentDirectory = ""): DocumentRegistry {
         // Maps from compiler setting target (ES3, ES5, etc.) to all the cached documents we have
         // for those settings.
-        const buckets: Map<FileMap<DocumentRegistryEntry>> = {};
+        const buckets = createMap<FileMap<DocumentRegistryEntry>>();
         const getCanonicalFileName = createGetCanonicalFileName(!!useCaseSensitiveFileNames);
 
         function getKeyForCompilationSettings(settings: CompilerOptions): DocumentRegistryBucketKey {
@@ -2251,7 +2253,7 @@ namespace ts {
         }
 
         function getBucketForCompilationSettings(key: DocumentRegistryBucketKey, createIfMissing: boolean): FileMap<DocumentRegistryEntry> {
-            let bucket = lookUp(buckets, key);
+            let bucket = buckets[key];
             if (!bucket && createIfMissing) {
                 buckets[key] = bucket = createFileMap<DocumentRegistryEntry>();
             }
@@ -2260,7 +2262,7 @@ namespace ts {
 
         function reportStats() {
             const bucketInfoArray = Object.keys(buckets).filter(name => name && name.charAt(0) === "_").map(name => {
-                const entries = lookUp(buckets, name);
+                const entries = buckets[name];
                 const sourceFiles: { name: string; refCount: number; references: string[]; }[] = [];
                 entries.forEachValue((key, entry) => {
                     sourceFiles.push({
@@ -3099,7 +3101,7 @@ namespace ts {
                  oldSettings.allowJs !== newSettings.allowJs ||
                  oldSettings.disableSizeLimit !== oldSettings.disableSizeLimit ||
                  oldSettings.baseUrl !== newSettings.baseUrl ||
-                 !mapIsEqualTo(oldSettings.paths, newSettings.paths));
+                 !equalOwnProperties(oldSettings.paths, newSettings.paths));
 
             // Now create a new compiler
             const compilerHost: CompilerHost = {
@@ -3114,7 +3116,6 @@ namespace ts {
                 getCurrentDirectory: () => currentDirectory,
                 fileExists: (fileName): boolean => {
                     // stub missing host functionality
-                    Debug.assert(!host.resolveModuleNames || !host.resolveTypeReferenceDirectives);
                     return hostCache.getOrCreateEntry(fileName) !== undefined;
                 },
                 readFile: (fileName): string => {
@@ -4102,7 +4103,7 @@ namespace ts {
              *          do not occur at the current position and have not otherwise been typed.
              */
             function filterNamedImportOrExportCompletionItems(exportsOfModule: Symbol[], namedImportsOrExports: ImportOrExportSpecifier[]): Symbol[] {
-                const existingImportsOrExports: Map<boolean> = {};
+                const existingImportsOrExports = createMap<boolean>();
 
                 for (const element of namedImportsOrExports) {
                     // If this is the current item we are editing right now, do not filter it out
@@ -4114,11 +4115,11 @@ namespace ts {
                     existingImportsOrExports[name.text] = true;
                 }
 
-                if (isEmpty(existingImportsOrExports)) {
+                if (!someProperties(existingImportsOrExports)) {
                     return filter(exportsOfModule, e => e.name !== "default");
                 }
 
-                return filter(exportsOfModule, e => e.name !== "default" && !lookUp(existingImportsOrExports, e.name));
+                return filter(exportsOfModule, e => e.name !== "default" && !existingImportsOrExports[e.name]);
             }
 
             /**
@@ -4132,7 +4133,7 @@ namespace ts {
                     return contextualMemberSymbols;
                 }
 
-                const existingMemberNames: Map<boolean> = {};
+                const existingMemberNames = createMap<boolean>();
                 for (const m of existingMembers) {
                     // Ignore omitted expressions for missing members
                     if (m.kind !== SyntaxKind.PropertyAssignment &&
@@ -4165,7 +4166,7 @@ namespace ts {
                     existingMemberNames[existingName] = true;
                 }
 
-                return filter(contextualMemberSymbols, m => !lookUp(existingMemberNames, m.name));
+                return filter(contextualMemberSymbols, m => !existingMemberNames[m.name]);
             }
 
             /**
@@ -4175,7 +4176,7 @@ namespace ts {
              *          do not occur at the current position and have not otherwise been typed.
              */
             function filterJsxAttributes(symbols: Symbol[], attributes: NodeArray<JsxAttribute | JsxSpreadAttribute>): Symbol[] {
-                const seenNames: Map<boolean> = {};
+                const seenNames = createMap<boolean>();
                 for (const attr of attributes) {
                     // If this is the current item we are editing right now, do not filter it out
                     if (attr.getStart() <= position && position <= attr.getEnd()) {
@@ -4187,7 +4188,7 @@ namespace ts {
                     }
                 }
 
-                return filter(symbols, a => !lookUp(seenNames, a.name));
+                return filter(symbols, a => !seenNames[a.name]);
             }
         }
 
@@ -4317,13 +4318,13 @@ namespace ts {
 
             function getCompletionEntriesFromSymbols(symbols: Symbol[], entries: CompletionEntry[], location: Node, performCharacterChecks: boolean): Map<string> {
                 const start = timestamp();
-                const uniqueNames: Map<string> = {};
+                const uniqueNames = createMap<string>();
                 if (symbols) {
                     for (const symbol of symbols) {
                         const entry = createCompletionEntry(symbol, location, performCharacterChecks);
                         if (entry) {
                             const id = escapeIdentifier(entry.name);
-                            if (!lookUp(uniqueNames, id)) {
+                            if (!uniqueNames[id]) {
                                 entries.push(entry);
                                 uniqueNames[id] = id;
                             }
@@ -4830,7 +4831,14 @@ namespace ts {
             }
             if (symbolFlags & SymbolFlags.Alias) {
                 addNewLineIfDisplayPartsExist();
-                displayParts.push(keywordPart(SyntaxKind.ImportKeyword));
+                if (symbol.declarations[0].kind === SyntaxKind.NamespaceExportDeclaration) {
+                    displayParts.push(keywordPart(SyntaxKind.ExportKeyword));
+                    displayParts.push(spacePart());
+                    displayParts.push(keywordPart(SyntaxKind.NamespaceKeyword));
+                }
+                else {
+                    displayParts.push(keywordPart(SyntaxKind.ImportKeyword));
+                }
                 displayParts.push(spacePart());
                 addFullSymbolName(symbol);
                 ts.forEach(symbol.declarations, declaration => {
@@ -5144,7 +5152,7 @@ namespace ts {
             // Type reference directives
             const typeReferenceDirective = findReferenceInPosition(sourceFile.typeReferenceDirectives, position);
             if (typeReferenceDirective) {
-                const referenceFile = lookUp(program.getResolvedTypeReferenceDirectives(), typeReferenceDirective.fileName);
+                const referenceFile = program.getResolvedTypeReferenceDirectives()[typeReferenceDirective.fileName];
                 if (referenceFile && referenceFile.resolvedFileName) {
                     return [getDefinitionInfoForFileReference(typeReferenceDirective.fileName, referenceFile.resolvedFileName)];
                 }
@@ -5311,12 +5319,12 @@ namespace ts {
                         return undefined;
                     }
 
-                    const fileNameToDocumentHighlights: Map<DocumentHighlights> = {};
+                    const fileNameToDocumentHighlights = createMap<DocumentHighlights>();
                     const result: DocumentHighlights[] = [];
                     for (const referencedSymbol of referencedSymbols) {
                         for (const referenceEntry of referencedSymbol.references) {
                             const fileName = referenceEntry.fileName;
-                            let documentHighlights = getProperty(fileNameToDocumentHighlights, fileName);
+                            let documentHighlights = fileNameToDocumentHighlights[fileName];
                             if (!documentHighlights) {
                                 documentHighlights = { fileName, highlightSpans: [] };
 
@@ -6061,7 +6069,7 @@ namespace ts {
 
                     const nameTable = getNameTable(sourceFile);
 
-                    if (lookUp(nameTable, internedName) !== undefined) {
+                    if (nameTable[internedName] !== undefined) {
                         result = result || [];
                         getReferencesInNode(sourceFile, symbol, declaredName, node, searchMeaning, findInStrings, findInComments, result, symbolToIndex);
                     }
@@ -6706,7 +6714,7 @@ namespace ts {
 
                     // Add symbol of properties/methods of the same name in base classes and implemented interfaces definitions
                     if (rootSymbol.parent && rootSymbol.parent.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
-                        getPropertySymbolsFromBaseTypes(rootSymbol.parent, rootSymbol.getName(), result, /*previousIterationSymbolsCache*/ {});
+                        getPropertySymbolsFromBaseTypes(rootSymbol.parent, rootSymbol.getName(), result, /*previousIterationSymbolsCache*/ createMap<Symbol>());
                     }
                 });
 
@@ -6738,7 +6746,7 @@ namespace ts {
                 // the function will add any found symbol of the property-name, then its sub-routine will call
                 // getPropertySymbolsFromBaseTypes again to walk up any base types to prevent revisiting already
                 // visited symbol, interface "C", the sub-routine will pass the current symbol as previousIterationSymbol.
-                if (hasProperty(previousIterationSymbolsCache, symbol.name)) {
+                if (symbol.name in previousIterationSymbolsCache) {
                     return;
                 }
 
@@ -6827,7 +6835,7 @@ namespace ts {
                     // see if any is in the list
                     if (rootSymbol.parent && rootSymbol.parent.flags & (SymbolFlags.Class | SymbolFlags.Interface)) {
                         const result: Symbol[] = [];
-                        getPropertySymbolsFromBaseTypes(rootSymbol.parent, rootSymbol.getName(), result, /*previousIterationSymbolsCache*/ {});
+                        getPropertySymbolsFromBaseTypes(rootSymbol.parent, rootSymbol.getName(), result, /*previousIterationSymbolsCache*/ createMap<Symbol>());
                         return forEach(result, s => searchSymbols.indexOf(s) >= 0 ? s : undefined);
                     }
 
@@ -8311,7 +8319,7 @@ namespace ts {
     }
 
     function initializeNameTable(sourceFile: SourceFile): void {
-        const nameTable: Map<number> = {};
+        const nameTable = createMap<number>();
 
         walk(sourceFile);
         sourceFile.nameTable = nameTable;
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 271d6f2d6e2..ff57dd9cf7a 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -72,8 +72,13 @@ namespace ts {
         directoryExists(directoryName: string): boolean;
     }
 
-    /** Public interface of the the of a config service shim instance.*/
-    export interface CoreServicesShimHost extends Logger, ModuleResolutionHost {
+    /** Public interface of the core-services host instance used in managed side */
+    export interface CoreServicesShimHost extends Logger {
+        directoryExists(directoryName: string): boolean;
+        fileExists(fileName: string): boolean;
+        getCurrentDirectory(): string;
+        getDirectories(path: string): string;
+
         /**
          * Returns a JSON-encoded value of the type: string[]
          *
@@ -81,9 +86,14 @@ namespace ts {
          *  when enumerating the directory.
          */
         readDirectory(rootDir: string, extension: string, basePaths?: string, excludeEx?: string, includeFileEx?: string, includeDirEx?: string, depth?: number): string;
-        useCaseSensitiveFileNames?(): boolean;
-        getCurrentDirectory(): string;
+
+        /**
+         * Read arbitary text files on disk, i.e. when resolution procedure needs the content of 'package.json' to determine location of bundled typings for node modules
+         */
+        readFile(fileName: string): string;
+        realpath?(path: string): string;
         trace(s: string): void;
+        useCaseSensitiveFileNames?(): boolean;
     }
 
     ///
@@ -240,6 +250,7 @@ namespace ts {
     }
 
     export interface CoreServicesShim extends Shim {
+        getAutomaticTypeDirectiveNames(compilerOptionsJson: string): string;
         getPreProcessedFileInfo(fileName: string, sourceText: IScriptSnapshot): string;
         getTSConfigFileInfo(fileName: string, sourceText: IScriptSnapshot): string;
         getDefaultCompilationSettings(): string;
@@ -300,9 +311,9 @@ namespace ts {
             // 'in' does not have this effect.
             if ("getModuleResolutionsForFile" in this.shimHost) {
                 this.resolveModuleNames = (moduleNames: string[], containingFile: string) => {
-                    const resolutionsInFile = <Map<string>>JSON.parse(this.shimHost.getModuleResolutionsForFile(containingFile));
+                    const resolutionsInFile = <MapLike<string>>JSON.parse(this.shimHost.getModuleResolutionsForFile(containingFile));
                     return map(moduleNames, name => {
-                        const result = lookUp(resolutionsInFile, name);
+                        const result = getProperty(resolutionsInFile, name);
                         return result ? { resolvedFileName: result } : undefined;
                     });
                 };
@@ -312,8 +323,8 @@ namespace ts {
             }
             if ("getTypeReferenceDirectiveResolutionsForFile" in this.shimHost) {
                 this.resolveTypeReferenceDirectives = (typeDirectiveNames: string[], containingFile: string) => {
-                    const typeDirectivesForFile = <Map<ResolvedTypeReferenceDirective>>JSON.parse(this.shimHost.getTypeReferenceDirectiveResolutionsForFile(containingFile));
-                    return map(typeDirectiveNames, name => lookUp(typeDirectivesForFile, name));
+                    const typeDirectivesForFile = <MapLike<ResolvedTypeReferenceDirective>>JSON.parse(this.shimHost.getTypeReferenceDirectiveResolutionsForFile(containingFile));
+                    return map(typeDirectiveNames, name => getProperty(typeDirectivesForFile, name));
                 };
             }
         }
@@ -492,6 +503,10 @@ namespace ts {
         private readDirectoryFallback(rootDir: string, extension: string, exclude: string[]) {
             return JSON.parse(this.shimHost.readDirectory(rootDir, extension, JSON.stringify(exclude)));
         }
+
+        public getDirectories(path: string): string[] {
+            return JSON.parse(this.shimHost.getDirectories(path));
+        }
     }
 
     function simpleForwardCall(logger: Logger, actionDescription: string, action: () => any, logPerformance: boolean): any {
@@ -1003,7 +1018,7 @@ namespace ts {
 
         public getPreProcessedFileInfo(fileName: string, sourceTextSnapshot: IScriptSnapshot): string {
             return this.forwardJSONCall(
-                "getPreProcessedFileInfo('" + fileName + "')",
+                `getPreProcessedFileInfo('${fileName}')`,
                 () => {
                     // for now treat files as JavaScript
                     const result = preProcessFile(sourceTextSnapshot.getText(0, sourceTextSnapshot.getLength()), /* readImportFiles */ true, /* detectJavaScriptImports */ true);
@@ -1017,6 +1032,16 @@ namespace ts {
                 });
         }
 
+        public getAutomaticTypeDirectiveNames(compilerOptionsJson: string): string {
+            return this.forwardJSONCall(
+                `getAutomaticTypeDirectiveNames('${compilerOptionsJson}')`,
+                () => {
+                    const compilerOptions = <CompilerOptions>JSON.parse(compilerOptionsJson);
+                    return getAutomaticTypeDirectiveNames(compilerOptions, this.host);
+                }
+            );
+        }
+
         private convertFileReferences(refs: FileReference[]): IFileReference[] {
             if (!refs) {
                 return undefined;
diff --git a/src/services/signatureHelp.ts b/src/services/signatureHelp.ts
index 50378aa64b1..211e55b23ba 100644
--- a/src/services/signatureHelp.ts
+++ b/src/services/signatureHelp.ts
@@ -237,7 +237,7 @@ namespace ts.SignatureHelp {
         const typeChecker = program.getTypeChecker();
         for (const sourceFile of program.getSourceFiles()) {
             const nameToDeclarations = sourceFile.getNamedDeclarations();
-            const declarations = getProperty(nameToDeclarations, name.text);
+            const declarations = nameToDeclarations[name.text];
 
             if (declarations) {
                 for (const declaration of declarations) {
diff --git a/tests/baselines/reference/APISample_watcher.js b/tests/baselines/reference/APISample_watcher.js
index 7573f2ba99e..cb7fc3a4c5c 100644
--- a/tests/baselines/reference/APISample_watcher.js
+++ b/tests/baselines/reference/APISample_watcher.js
@@ -20,7 +20,7 @@ declare var path: any;
 import * as ts from "typescript";
 
 function watch(rootFileNames: string[], options: ts.CompilerOptions) {
-    const files: ts.Map<{ version: number }> = {};
+    const files: ts.MapLike<{ version: number }> = {};
 
     // initialize the list of files
     rootFileNames.forEach(fileName => {
diff --git a/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt
new file mode 100644
index 00000000000..6f4a0ef9d6d
--- /dev/null
+++ b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt
@@ -0,0 +1,46 @@
+tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(13,1): error TS2322: Type 'A[]' is not assignable to type 'ReadonlyArray<B>'.
+  Types of property 'concat' are incompatible.
+    Type '{ (...items: A[][]): A[]; (...items: (A | A[])[]): A[]; }' is not assignable to type '{ <U extends ReadonlyArray<B>>(...items: U[]): B[]; (...items: B[][]): B[]; (...items: (B | B[])[]): B[]; }'.
+      Type 'A[]' is not assignable to type 'B[]'.
+        Type 'A' is not assignable to type 'B'.
+          Property 'b' is missing in type 'A'.
+tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error TS2322: Type 'C<A>' is not assignable to type 'ReadonlyArray<B>'.
+  Types of property 'concat' are incompatible.
+    Type '{ (...items: A[][]): A[]; (...items: (A | A[])[]): A[]; }' is not assignable to type '{ <U extends ReadonlyArray<B>>(...items: U[]): B[]; (...items: B[][]): B[]; (...items: (B | B[])[]): B[]; }'.
+      Type 'A[]' is not assignable to type 'B[]'.
+        Type 'A' is not assignable to type 'B'.
+
+
+==== tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts (2 errors) ====
+    class A { a }
+    class B extends A { b }
+    class C<T> extends Array<T> { c }
+    declare var ara: A[];
+    declare var arb: B[];
+    declare var cra: C<A>;
+    declare var crb: C<B>;
+    declare var rra: ReadonlyArray<A>;
+    declare var rrb: ReadonlyArray<B>;
+    rra = ara;
+    rrb = arb; // OK, Array<B> is assignable to ReadonlyArray<A>
+    rra = arb;
+    rrb = ara; // error: 'A' is not assignable to 'B'
+    ~~~
+!!! error TS2322: Type 'A[]' is not assignable to type 'ReadonlyArray<B>'.
+!!! error TS2322:   Types of property 'concat' are incompatible.
+!!! error TS2322:     Type '{ (...items: A[][]): A[]; (...items: (A | A[])[]): A[]; }' is not assignable to type '{ <U extends ReadonlyArray<B>>(...items: U[]): B[]; (...items: B[][]): B[]; (...items: (B | B[])[]): B[]; }'.
+!!! error TS2322:       Type 'A[]' is not assignable to type 'B[]'.
+!!! error TS2322:         Type 'A' is not assignable to type 'B'.
+!!! error TS2322:           Property 'b' is missing in type 'A'.
+    
+    rra = cra;
+    rra = crb; // OK, C<B> is assignable to ReadonlyArray<A>
+    rrb = crb;
+    rrb = cra; // error: 'A' is not assignable to 'B'
+    ~~~
+!!! error TS2322: Type 'C<A>' is not assignable to type 'ReadonlyArray<B>'.
+!!! error TS2322:   Types of property 'concat' are incompatible.
+!!! error TS2322:     Type '{ (...items: A[][]): A[]; (...items: (A | A[])[]): A[]; }' is not assignable to type '{ <U extends ReadonlyArray<B>>(...items: U[]): B[]; (...items: B[][]): B[]; (...items: (B | B[])[]): B[]; }'.
+!!! error TS2322:       Type 'A[]' is not assignable to type 'B[]'.
+!!! error TS2322:         Type 'A' is not assignable to type 'B'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.js b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.js
new file mode 100644
index 00000000000..255b52e16c9
--- /dev/null
+++ b/tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.js
@@ -0,0 +1,54 @@
+//// [arrayOfSubtypeIsAssignableToReadonlyArray.ts]
+class A { a }
+class B extends A { b }
+class C<T> extends Array<T> { c }
+declare var ara: A[];
+declare var arb: B[];
+declare var cra: C<A>;
+declare var crb: C<B>;
+declare var rra: ReadonlyArray<A>;
+declare var rrb: ReadonlyArray<B>;
+rra = ara;
+rrb = arb; // OK, Array<B> is assignable to ReadonlyArray<A>
+rra = arb;
+rrb = ara; // error: 'A' is not assignable to 'B'
+
+rra = cra;
+rra = crb; // OK, C<B> is assignable to ReadonlyArray<A>
+rrb = crb;
+rrb = cra; // error: 'A' is not assignable to 'B'
+
+
+//// [arrayOfSubtypeIsAssignableToReadonlyArray.js]
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var A = (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = (function (_super) {
+    __extends(B, _super);
+    function B() {
+        _super.apply(this, arguments);
+    }
+    return B;
+}(A));
+var C = (function (_super) {
+    __extends(C, _super);
+    function C() {
+        _super.apply(this, arguments);
+    }
+    return C;
+}(Array));
+rra = ara;
+rrb = arb; // OK, Array<B> is assignable to ReadonlyArray<A>
+rra = arb;
+rrb = ara; // error: 'A' is not assignable to 'B'
+rra = cra;
+rra = crb; // OK, C<B> is assignable to ReadonlyArray<A>
+rrb = crb;
+rrb = cra; // error: 'A' is not assignable to 'B'
diff --git a/tests/baselines/reference/classExpressionES63.js b/tests/baselines/reference/classExpressionES63.js
index 6b3a06cf7c3..5a72e173475 100644
--- a/tests/baselines/reference/classExpressionES63.js
+++ b/tests/baselines/reference/classExpressionES63.js
@@ -13,14 +13,14 @@ let C = class extends class extends class {
     }
 }
  {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.b = 2;
     }
 }
  {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.c = 3;
     }
 }
diff --git a/tests/baselines/reference/controlFlowBinaryOrExpression.symbols b/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
index e76cfbf4cab..217e11dc95d 100644
--- a/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
+++ b/tests/baselines/reference/controlFlowBinaryOrExpression.symbols
@@ -64,9 +64,9 @@ if (isNodeList(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(NodeList.length, Decl(controlFlowBinaryOrExpression.ts, 10, 27))
 }
 
 if (isHTMLCollection(sourceObj)) {
@@ -74,9 +74,9 @@ if (isHTMLCollection(sourceObj)) {
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
 
     sourceObj.length;
->sourceObj.length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>sourceObj.length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 >sourceObj : Symbol(sourceObj, Decl(controlFlowBinaryOrExpression.ts, 23, 3))
->length : Symbol(length, Decl(controlFlowBinaryOrExpression.ts, 10, 27), Decl(controlFlowBinaryOrExpression.ts, 14, 33))
+>length : Symbol(HTMLCollection.length, Decl(controlFlowBinaryOrExpression.ts, 14, 33))
 }
 
 if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
diff --git a/tests/baselines/reference/controlFlowBinaryOrExpression.types b/tests/baselines/reference/controlFlowBinaryOrExpression.types
index 0bf5ab524b9..7e92fcdbbd1 100644
--- a/tests/baselines/reference/controlFlowBinaryOrExpression.types
+++ b/tests/baselines/reference/controlFlowBinaryOrExpression.types
@@ -80,7 +80,7 @@ if (isNodeList(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : NodeList
 >length : number
 }
 
@@ -91,7 +91,7 @@ if (isHTMLCollection(sourceObj)) {
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection
+>sourceObj : HTMLCollection
 >length : number
 }
 
@@ -102,11 +102,11 @@ if (isNodeList(sourceObj) || isHTMLCollection(sourceObj)) {
 >sourceObj : EventTargetLike
 >isHTMLCollection(sourceObj) : boolean
 >isHTMLCollection : (sourceObj: any) => sourceObj is HTMLCollection
->sourceObj : { a: string; }
+>sourceObj : HTMLCollection | { a: string; }
 
     sourceObj.length;
 >sourceObj.length : number
->sourceObj : NodeList | HTMLCollection | ({ a: string; } & HTMLCollection)
+>sourceObj : NodeList | HTMLCollection
 >length : number
 }
 
diff --git a/tests/baselines/reference/controlFlowIfStatement.js b/tests/baselines/reference/controlFlowIfStatement.js
index a70c07d38dc..e40b831552f 100644
--- a/tests/baselines/reference/controlFlowIfStatement.js
+++ b/tests/baselines/reference/controlFlowIfStatement.js
@@ -35,6 +35,22 @@ function b() {
     }
     x; // string
 }
+function c<T>(data: string | T): T {
+    if (typeof data === 'string') {
+        return JSON.parse(data);
+    }
+    else {
+        return data;
+    }
+}
+function d<T extends string>(data: string | T): never {
+    if (typeof data === 'string') {
+        throw new Error('will always happen');
+    }
+    else {
+        return data;
+    }
+}
 
 
 //// [controlFlowIfStatement.js]
@@ -72,3 +88,19 @@ function b() {
     }
     x; // string
 }
+function c(data) {
+    if (typeof data === 'string') {
+        return JSON.parse(data);
+    }
+    else {
+        return data;
+    }
+}
+function d(data) {
+    if (typeof data === 'string') {
+        throw new Error('will always happen');
+    }
+    else {
+        return data;
+    }
+}
diff --git a/tests/baselines/reference/controlFlowIfStatement.symbols b/tests/baselines/reference/controlFlowIfStatement.symbols
index 1d85b31c998..e4d2bb9f184 100644
--- a/tests/baselines/reference/controlFlowIfStatement.symbols
+++ b/tests/baselines/reference/controlFlowIfStatement.symbols
@@ -71,4 +71,42 @@ function b() {
     x; // string
 >x : Symbol(x, Decl(controlFlowIfStatement.ts, 26, 7))
 }
+function c<T>(data: string | T): T {
+>c : Symbol(c, Decl(controlFlowIfStatement.ts, 35, 1))
+>T : Symbol(T, Decl(controlFlowIfStatement.ts, 36, 11))
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 36, 14))
+>T : Symbol(T, Decl(controlFlowIfStatement.ts, 36, 11))
+>T : Symbol(T, Decl(controlFlowIfStatement.ts, 36, 11))
+
+    if (typeof data === 'string') {
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 36, 14))
+
+        return JSON.parse(data);
+>JSON.parse : Symbol(JSON.parse, Decl(lib.d.ts, --, --))
+>JSON : Symbol(JSON, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>parse : Symbol(JSON.parse, Decl(lib.d.ts, --, --))
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 36, 14))
+    }
+    else {
+        return data;
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 36, 14))
+    }
+}
+function d<T extends string>(data: string | T): never {
+>d : Symbol(d, Decl(controlFlowIfStatement.ts, 43, 1))
+>T : Symbol(T, Decl(controlFlowIfStatement.ts, 44, 11))
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 44, 29))
+>T : Symbol(T, Decl(controlFlowIfStatement.ts, 44, 11))
+
+    if (typeof data === 'string') {
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 44, 29))
+
+        throw new Error('will always happen');
+>Error : Symbol(Error, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+    }
+    else {
+        return data;
+>data : Symbol(data, Decl(controlFlowIfStatement.ts, 44, 29))
+    }
+}
 
diff --git a/tests/baselines/reference/controlFlowIfStatement.types b/tests/baselines/reference/controlFlowIfStatement.types
index 716491ea58b..79985378bf5 100644
--- a/tests/baselines/reference/controlFlowIfStatement.types
+++ b/tests/baselines/reference/controlFlowIfStatement.types
@@ -90,4 +90,51 @@ function b() {
     x; // string
 >x : string
 }
+function c<T>(data: string | T): T {
+>c : <T>(data: string | T) => T
+>T : T
+>data : string | T
+>T : T
+>T : T
+
+    if (typeof data === 'string') {
+>typeof data === 'string' : boolean
+>typeof data : string
+>data : string | T
+>'string' : "string"
+
+        return JSON.parse(data);
+>JSON.parse(data) : any
+>JSON.parse : (text: string, reviver?: (key: any, value: any) => any) => any
+>JSON : JSON
+>parse : (text: string, reviver?: (key: any, value: any) => any) => any
+>data : string
+    }
+    else {
+        return data;
+>data : T
+    }
+}
+function d<T extends string>(data: string | T): never {
+>d : <T extends string>(data: string | T) => never
+>T : T
+>data : string | T
+>T : T
+
+    if (typeof data === 'string') {
+>typeof data === 'string' : boolean
+>typeof data : string
+>data : string | T
+>'string' : "string"
+
+        throw new Error('will always happen');
+>new Error('will always happen') : Error
+>Error : ErrorConstructor
+>'will always happen' : string
+    }
+    else {
+        return data;
+>data : never
+    }
+}
 
diff --git a/tests/baselines/reference/controlFlowInstanceof.js b/tests/baselines/reference/controlFlowInstanceof.js
new file mode 100644
index 00000000000..3f2287b4628
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.js
@@ -0,0 +1,184 @@
+//// [controlFlowInstanceof.ts]
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f2(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Promise) {
+        s;  // Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f3(s: Set<string> | Set<number>) {
+    s;  // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s;  // Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+// More tests
+
+class A { a: string }
+class B extends A { b: string }
+class C extends A { c: string }
+
+function foo(x: A | undefined) {
+    x;  // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x;  // B | C
+    }
+    x;  // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x;  // B & C
+    }
+    x;  // A | undefined
+    if (!x) {
+        return;
+    }
+    x;  // A
+    if (x instanceof B) {
+        x;  // B
+        if (x instanceof C) {
+            x;  // B & C
+        }
+        else {
+            x;  // B
+        }
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+    x;  // A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+    x?: string;
+}
+
+class Y {
+    y: string;
+}
+
+function goo(x: X) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
+
+//// [controlFlowInstanceof.js]
+// Repros from #10167
+function f1(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Set) {
+        s; // Set<number>
+    }
+    s; // Set<number>
+    s.add(42);
+}
+function f2(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Promise) {
+        s; // Set<number> & Promise<any>
+    }
+    s; // Set<number>
+    s.add(42);
+}
+function f3(s) {
+    s; // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s; // Set<string> | Set<number>
+    }
+    else {
+        s; // never
+    }
+}
+function f4(s) {
+    s = new Set();
+    s; // Set<number>
+    if (s instanceof Set) {
+        s; // Set<number>
+    }
+    else {
+        s; // never
+    }
+}
+// More tests
+class A {
+}
+class B extends A {
+}
+class C extends A {
+}
+function foo(x) {
+    x; // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x; // B | C
+    }
+    x; // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x; // B & C
+    }
+    x; // A | undefined
+    if (!x) {
+        return;
+    }
+    x; // A
+    if (x instanceof B) {
+        x; // B
+        if (x instanceof C) {
+            x; // B & C
+        }
+        else {
+            x; // B
+        }
+        x; // B
+    }
+    else {
+        x; // A
+    }
+    x; // A
+}
+class Y {
+}
+function goo(x) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
diff --git a/tests/baselines/reference/controlFlowInstanceof.symbols b/tests/baselines/reference/controlFlowInstanceof.symbols
new file mode 100644
index 00000000000..31894827990
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.symbols
@@ -0,0 +1,232 @@
+=== tests/cases/compiler/controlFlowInstanceof.ts ===
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+>f1 : Symbol(f1, Decl(controlFlowInstanceof.ts, 0, 0))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+    }
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+
+    s.add(42);
+>s.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 3, 12))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+}
+
+function f2(s: Set<string> | Set<number>) {
+>f2 : Symbol(f2, Decl(controlFlowInstanceof.ts, 11, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+
+    if (s instanceof Promise) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>Promise : Symbol(Promise, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.promise.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --))
+
+        s;  // Set<number> & Promise<any>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+    }
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+
+    s.add(42);
+>s.add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 13, 12))
+>add : Symbol(Set.add, Decl(lib.es2015.collection.d.ts, --, --))
+}
+
+function f3(s: Set<string> | Set<number>) {
+>f3 : Symbol(f3, Decl(controlFlowInstanceof.ts, 21, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<string> | Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<string> | Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+    }
+    else {
+        s;  // never
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 23, 12))
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+>f4 : Symbol(f4, Decl(controlFlowInstanceof.ts, 31, 1))
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s = new Set<number>();
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+    s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+
+    if (s instanceof Set) {
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+>Set : Symbol(Set, Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --), Decl(lib.es2015.collection.d.ts, --, --))
+
+        s;  // Set<number>
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+    }
+    else {
+        s;  // never
+>s : Symbol(s, Decl(controlFlowInstanceof.ts, 33, 12))
+    }
+}
+
+// More tests
+
+class A { a: string }
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>a : Symbol(A.a, Decl(controlFlowInstanceof.ts, 46, 9))
+
+class B extends A { b: string }
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>b : Symbol(B.b, Decl(controlFlowInstanceof.ts, 47, 19))
+
+class C extends A { c: string }
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+>c : Symbol(C.c, Decl(controlFlowInstanceof.ts, 48, 19))
+
+function foo(x: A | undefined) {
+>foo : Symbol(foo, Decl(controlFlowInstanceof.ts, 48, 31))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>A : Symbol(A, Decl(controlFlowInstanceof.ts, 42, 1))
+
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B || x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+        x;  // B | C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B && x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+        x;  // B & C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A | undefined
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (!x) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+        return;
+    }
+    x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+    if (x instanceof B) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>B : Symbol(B, Decl(controlFlowInstanceof.ts, 46, 21))
+
+        x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+
+        if (x instanceof C) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+>C : Symbol(C, Decl(controlFlowInstanceof.ts, 47, 31))
+
+            x;  // B & C
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+        }
+        else {
+            x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+        }
+        x;  // B
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    else {
+        x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+    }
+    x;  // A
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 50, 13))
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+>X : Symbol(X, Decl(controlFlowInstanceof.ts, 78, 1))
+
+    x?: string;
+>x : Symbol(X.x, Decl(controlFlowInstanceof.ts, 83, 13))
+}
+
+class Y {
+>Y : Symbol(Y, Decl(controlFlowInstanceof.ts, 85, 1))
+
+    y: string;
+>y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+}
+
+function goo(x: X) {
+>goo : Symbol(goo, Decl(controlFlowInstanceof.ts, 89, 1))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>X : Symbol(X, Decl(controlFlowInstanceof.ts, 78, 1))
+
+    x;
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+
+    if (x instanceof Y) {
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>Y : Symbol(Y, Decl(controlFlowInstanceof.ts, 85, 1))
+
+        x.y;
+>x.y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+>y : Symbol(Y.y, Decl(controlFlowInstanceof.ts, 87, 9))
+    }
+    x;
+>x : Symbol(x, Decl(controlFlowInstanceof.ts, 91, 13))
+}
diff --git a/tests/baselines/reference/controlFlowInstanceof.types b/tests/baselines/reference/controlFlowInstanceof.types
new file mode 100644
index 00000000000..e52d1a25b1d
--- /dev/null
+++ b/tests/baselines/reference/controlFlowInstanceof.types
@@ -0,0 +1,256 @@
+=== tests/cases/compiler/controlFlowInstanceof.ts ===
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+>f1 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<number>
+>Set : SetConstructor
+
+        s;  // Set<number>
+>s : Set<number>
+    }
+    s;  // Set<number>
+>s : Set<number>
+
+    s.add(42);
+>s.add(42) : Set<number>
+>s.add : (value: number) => Set<number>
+>s : Set<number>
+>add : (value: number) => Set<number>
+>42 : number
+}
+
+function f2(s: Set<string> | Set<number>) {
+>f2 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Promise) {
+>s instanceof Promise : boolean
+>s : Set<number>
+>Promise : PromiseConstructor
+
+        s;  // Set<number> & Promise<any>
+>s : Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+>s : Set<number>
+
+    s.add(42);
+>s.add(42) : Set<number>
+>s.add : (value: number) => Set<number>
+>s : Set<number>
+>add : (value: number) => Set<number>
+>42 : number
+}
+
+function f3(s: Set<string> | Set<number>) {
+>f3 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s;  // Set<string> | Set<number>
+>s : Set<string> | Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<string> | Set<number>
+>Set : SetConstructor
+
+        s;  // Set<string> | Set<number>
+>s : Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+>s : never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+>f4 : (s: Set<string> | Set<number>) => void
+>s : Set<string> | Set<number>
+>Set : Set<T>
+>Set : Set<T>
+
+    s = new Set<number>();
+>s = new Set<number>() : Set<number>
+>s : Set<string> | Set<number>
+>new Set<number>() : Set<number>
+>Set : SetConstructor
+
+    s;  // Set<number>
+>s : Set<number>
+
+    if (s instanceof Set) {
+>s instanceof Set : boolean
+>s : Set<number>
+>Set : SetConstructor
+
+        s;  // Set<number>
+>s : Set<number>
+    }
+    else {
+        s;  // never
+>s : never
+    }
+}
+
+// More tests
+
+class A { a: string }
+>A : A
+>a : string
+
+class B extends A { b: string }
+>B : B
+>A : A
+>b : string
+
+class C extends A { c: string }
+>C : C
+>A : A
+>c : string
+
+function foo(x: A | undefined) {
+>foo : (x: A) => void
+>x : A
+>A : A
+
+    x;  // A | undefined
+>x : A
+
+    if (x instanceof B || x instanceof C) {
+>x instanceof B || x instanceof C : boolean
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+>x instanceof C : boolean
+>x : A
+>C : typeof C
+
+        x;  // B | C
+>x : B | C
+    }
+    x;  // A | undefined
+>x : A
+
+    if (x instanceof B && x instanceof C) {
+>x instanceof B && x instanceof C : boolean
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+>x instanceof C : boolean
+>x : B
+>C : typeof C
+
+        x;  // B & C
+>x : B & C
+    }
+    x;  // A | undefined
+>x : A
+
+    if (!x) {
+>!x : boolean
+>x : A
+
+        return;
+    }
+    x;  // A
+>x : A
+
+    if (x instanceof B) {
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+
+        x;  // B
+>x : B
+
+        if (x instanceof C) {
+>x instanceof C : boolean
+>x : B
+>C : typeof C
+
+            x;  // B & C
+>x : B & C
+        }
+        else {
+            x;  // B
+>x : B
+        }
+        x;  // B
+>x : B
+    }
+    else {
+        x;  // A
+>x : A
+    }
+    x;  // A
+>x : A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+>X : X
+
+    x?: string;
+>x : string
+}
+
+class Y {
+>Y : Y
+
+    y: string;
+>y : string
+}
+
+function goo(x: X) {
+>goo : (x: X) => void
+>x : X
+>X : X
+
+    x;
+>x : X
+
+    if (x instanceof Y) {
+>x instanceof Y : boolean
+>x : X
+>Y : typeof Y
+
+        x.y;
+>x.y : string
+>x : Y
+>y : string
+    }
+    x;
+>x : X
+}
diff --git a/tests/baselines/reference/discriminantPropertyCheck.errors.txt b/tests/baselines/reference/discriminantPropertyCheck.errors.txt
new file mode 100644
index 00000000000..41593c4b224
--- /dev/null
+++ b/tests/baselines/reference/discriminantPropertyCheck.errors.txt
@@ -0,0 +1,77 @@
+tests/cases/compiler/discriminantPropertyCheck.ts(30,9): error TS2532: Object is possibly 'undefined'.
+tests/cases/compiler/discriminantPropertyCheck.ts(66,9): error TS2532: Object is possibly 'undefined'.
+
+
+==== tests/cases/compiler/discriminantPropertyCheck.ts (2 errors) ====
+    
+    type Item = Item1 | Item2;
+    
+    interface Base {
+        bar: boolean;
+    }
+    
+    interface Item1 extends Base {
+        kind: "A";
+        foo: string | undefined;
+        baz: boolean;
+        qux: true;
+    }
+    
+    interface Item2 extends Base {
+        kind: "B";
+        foo: string | undefined;
+        baz: boolean;
+        qux: false;
+    }
+    
+    function goo1(x: Item) {
+        if (x.kind === "A" && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function goo2(x: Item) {
+        if (x.foo !== undefined && x.kind === "A") {
+            x.foo.length;  // Error, intervening discriminant guard
+            ~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+    }
+    
+    function foo1(x: Item) {
+        if (x.bar && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo2(x: Item) {
+        if (x.foo !== undefined && x.bar) {
+            x.foo.length;
+        }
+    }
+    
+    function foo3(x: Item) {
+        if (x.baz && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo4(x: Item) {
+        if (x.foo !== undefined && x.baz) {
+            x.foo.length;
+        }
+    }
+    
+    function foo5(x: Item) {
+        if (x.qux && x.foo !== undefined) {
+            x.foo.length;
+        }
+    }
+    
+    function foo6(x: Item) {
+        if (x.foo !== undefined && x.qux) {
+            x.foo.length;  // Error, intervening discriminant guard
+            ~~~~~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/discriminantPropertyCheck.js b/tests/baselines/reference/discriminantPropertyCheck.js
new file mode 100644
index 00000000000..b75a6277735
--- /dev/null
+++ b/tests/baselines/reference/discriminantPropertyCheck.js
@@ -0,0 +1,111 @@
+//// [discriminantPropertyCheck.ts]
+
+type Item = Item1 | Item2;
+
+interface Base {
+    bar: boolean;
+}
+
+interface Item1 extends Base {
+    kind: "A";
+    foo: string | undefined;
+    baz: boolean;
+    qux: true;
+}
+
+interface Item2 extends Base {
+    kind: "B";
+    foo: string | undefined;
+    baz: boolean;
+    qux: false;
+}
+
+function goo1(x: Item) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function goo2(x: Item) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+function foo1(x: Item) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo2(x: Item) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+
+function foo3(x: Item) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo4(x: Item) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+
+function foo5(x: Item) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo6(x: Item) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+//// [discriminantPropertyCheck.js]
+function goo1(x) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function goo2(x) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length; // Error, intervening discriminant guard
+    }
+}
+function foo1(x) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo2(x) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+function foo3(x) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo4(x) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+function foo5(x) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+function foo6(x) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length; // Error, intervening discriminant guard
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndNullOrUndefined.js b/tests/baselines/reference/discriminantsAndNullOrUndefined.js
new file mode 100644
index 00000000000..153950f8ab5
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndNullOrUndefined.js
@@ -0,0 +1,44 @@
+//// [discriminantsAndNullOrUndefined.ts]
+
+// Repro from #10228
+
+interface A { kind: 'A'; }
+interface B { kind: 'B'; }
+
+type C = A | B | undefined;
+
+function never(_: never): never {
+    throw new Error();
+}
+
+function useA(_: A): void { }
+function useB(_: B): void { }
+
+declare var c: C;
+
+if (c !== undefined) {
+    switch (c.kind) {
+        case 'A': useA(c); break;
+        case 'B': useB(c); break;
+        default: never(c);
+    }
+}
+
+//// [discriminantsAndNullOrUndefined.js]
+// Repro from #10228
+function never(_) {
+    throw new Error();
+}
+function useA(_) { }
+function useB(_) { }
+if (c !== undefined) {
+    switch (c.kind) {
+        case 'A':
+            useA(c);
+            break;
+        case 'B':
+            useB(c);
+            break;
+        default: never(c);
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndNullOrUndefined.symbols b/tests/baselines/reference/discriminantsAndNullOrUndefined.symbols
new file mode 100644
index 00000000000..3f95fcf3a3f
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndNullOrUndefined.symbols
@@ -0,0 +1,61 @@
+=== tests/cases/compiler/discriminantsAndNullOrUndefined.ts ===
+
+// Repro from #10228
+
+interface A { kind: 'A'; }
+>A : Symbol(A, Decl(discriminantsAndNullOrUndefined.ts, 0, 0))
+>kind : Symbol(A.kind, Decl(discriminantsAndNullOrUndefined.ts, 3, 13))
+
+interface B { kind: 'B'; }
+>B : Symbol(B, Decl(discriminantsAndNullOrUndefined.ts, 3, 26))
+>kind : Symbol(B.kind, Decl(discriminantsAndNullOrUndefined.ts, 4, 13))
+
+type C = A | B | undefined;
+>C : Symbol(C, Decl(discriminantsAndNullOrUndefined.ts, 4, 26))
+>A : Symbol(A, Decl(discriminantsAndNullOrUndefined.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndNullOrUndefined.ts, 3, 26))
+
+function never(_: never): never {
+>never : Symbol(never, Decl(discriminantsAndNullOrUndefined.ts, 6, 27))
+>_ : Symbol(_, Decl(discriminantsAndNullOrUndefined.ts, 8, 15))
+
+    throw new Error();
+>Error : Symbol(Error, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+}
+
+function useA(_: A): void { }
+>useA : Symbol(useA, Decl(discriminantsAndNullOrUndefined.ts, 10, 1))
+>_ : Symbol(_, Decl(discriminantsAndNullOrUndefined.ts, 12, 14))
+>A : Symbol(A, Decl(discriminantsAndNullOrUndefined.ts, 0, 0))
+
+function useB(_: B): void { }
+>useB : Symbol(useB, Decl(discriminantsAndNullOrUndefined.ts, 12, 29))
+>_ : Symbol(_, Decl(discriminantsAndNullOrUndefined.ts, 13, 14))
+>B : Symbol(B, Decl(discriminantsAndNullOrUndefined.ts, 3, 26))
+
+declare var c: C;
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+>C : Symbol(C, Decl(discriminantsAndNullOrUndefined.ts, 4, 26))
+
+if (c !== undefined) {
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+>undefined : Symbol(undefined)
+
+    switch (c.kind) {
+>c.kind : Symbol(kind, Decl(discriminantsAndNullOrUndefined.ts, 3, 13), Decl(discriminantsAndNullOrUndefined.ts, 4, 13))
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+>kind : Symbol(kind, Decl(discriminantsAndNullOrUndefined.ts, 3, 13), Decl(discriminantsAndNullOrUndefined.ts, 4, 13))
+
+        case 'A': useA(c); break;
+>useA : Symbol(useA, Decl(discriminantsAndNullOrUndefined.ts, 10, 1))
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+
+        case 'B': useB(c); break;
+>useB : Symbol(useB, Decl(discriminantsAndNullOrUndefined.ts, 12, 29))
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+
+        default: never(c);
+>never : Symbol(never, Decl(discriminantsAndNullOrUndefined.ts, 6, 27))
+>c : Symbol(c, Decl(discriminantsAndNullOrUndefined.ts, 15, 11))
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndNullOrUndefined.types b/tests/baselines/reference/discriminantsAndNullOrUndefined.types
new file mode 100644
index 00000000000..7a2918ab83b
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndNullOrUndefined.types
@@ -0,0 +1,68 @@
+=== tests/cases/compiler/discriminantsAndNullOrUndefined.ts ===
+
+// Repro from #10228
+
+interface A { kind: 'A'; }
+>A : A
+>kind : "A"
+
+interface B { kind: 'B'; }
+>B : B
+>kind : "B"
+
+type C = A | B | undefined;
+>C : C
+>A : A
+>B : B
+
+function never(_: never): never {
+>never : (_: never) => never
+>_ : never
+
+    throw new Error();
+>new Error() : Error
+>Error : ErrorConstructor
+}
+
+function useA(_: A): void { }
+>useA : (_: A) => void
+>_ : A
+>A : A
+
+function useB(_: B): void { }
+>useB : (_: B) => void
+>_ : B
+>B : B
+
+declare var c: C;
+>c : C
+>C : C
+
+if (c !== undefined) {
+>c !== undefined : boolean
+>c : C
+>undefined : undefined
+
+    switch (c.kind) {
+>c.kind : "A" | "B"
+>c : A | B
+>kind : "A" | "B"
+
+        case 'A': useA(c); break;
+>'A' : "A"
+>useA(c) : void
+>useA : (_: A) => void
+>c : A
+
+        case 'B': useB(c); break;
+>'B' : "B"
+>useB(c) : void
+>useB : (_: B) => void
+>c : B
+
+        default: never(c);
+>never(c) : never
+>never : (_: never) => never
+>c : never
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndPrimitives.js b/tests/baselines/reference/discriminantsAndPrimitives.js
new file mode 100644
index 00000000000..1d11781a707
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndPrimitives.js
@@ -0,0 +1,84 @@
+//// [discriminantsAndPrimitives.ts]
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+    kind: "foo";
+    name: string;
+}
+
+interface Bar {
+    kind: "bar";
+    length: string;
+}
+
+function f1(x: Foo | Bar | string) {
+    if (typeof x !== 'string') {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+    if (typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+    if (x && typeof x !== "string") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+    if (x && typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+//// [discriminantsAndPrimitives.js]
+// Repro from #10257 plus other tests
+function f1(x) {
+    if (typeof x !== 'string') {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f2(x) {
+    if (typeof x === "object") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f3(x) {
+    if (x && typeof x !== "string") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+function f4(x) {
+    if (x && typeof x === "object") {
+        switch (x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndPrimitives.symbols b/tests/baselines/reference/discriminantsAndPrimitives.symbols
new file mode 100644
index 00000000000..c84f32cd990
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndPrimitives.symbols
@@ -0,0 +1,117 @@
+=== tests/cases/compiler/discriminantsAndPrimitives.ts ===
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+
+    kind: "foo";
+>kind : Symbol(Foo.kind, Decl(discriminantsAndPrimitives.ts, 3, 15))
+
+    name: string;
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+}
+
+interface Bar {
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    kind: "bar";
+>kind : Symbol(Bar.kind, Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+    length: string;
+>length : Symbol(Bar.length, Decl(discriminantsAndPrimitives.ts, 9, 16))
+}
+
+function f1(x: Foo | Bar | string) {
+>f1 : Symbol(f1, Decl(discriminantsAndPrimitives.ts, 11, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (typeof x !== 'string') {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 13, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+>f2 : Symbol(f2, Decl(discriminantsAndPrimitives.ts, 20, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (typeof x === "object") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 22, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+>f3 : Symbol(f3, Decl(discriminantsAndPrimitives.ts, 29, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (x && typeof x !== "string") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 31, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+>f4 : Symbol(f4, Decl(discriminantsAndPrimitives.ts, 38, 1))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>Foo : Symbol(Foo, Decl(discriminantsAndPrimitives.ts, 0, 0))
+>Bar : Symbol(Bar, Decl(discriminantsAndPrimitives.ts, 6, 1))
+
+    if (x && typeof x === "object") {
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+
+        switch(x.kind) {
+>x.kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>kind : Symbol(kind, Decl(discriminantsAndPrimitives.ts, 3, 15), Decl(discriminantsAndPrimitives.ts, 8, 15))
+
+            case 'foo':
+                x.name;
+>x.name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+>x : Symbol(x, Decl(discriminantsAndPrimitives.ts, 40, 12))
+>name : Symbol(Foo.name, Decl(discriminantsAndPrimitives.ts, 4, 16))
+        }
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndPrimitives.types b/tests/baselines/reference/discriminantsAndPrimitives.types
new file mode 100644
index 00000000000..d3e1b70e599
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndPrimitives.types
@@ -0,0 +1,141 @@
+=== tests/cases/compiler/discriminantsAndPrimitives.ts ===
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+>Foo : Foo
+
+    kind: "foo";
+>kind : "foo"
+
+    name: string;
+>name : string
+}
+
+interface Bar {
+>Bar : Bar
+
+    kind: "bar";
+>kind : "bar"
+
+    length: string;
+>length : string
+}
+
+function f1(x: Foo | Bar | string) {
+>f1 : (x: string | Foo | Bar) => void
+>x : string | Foo | Bar
+>Foo : Foo
+>Bar : Bar
+
+    if (typeof x !== 'string') {
+>typeof x !== 'string' : boolean
+>typeof x : string
+>x : string | Foo | Bar
+>'string' : "string"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+>f2 : (x: string | Foo | Bar | undefined) => void
+>x : string | Foo | Bar | undefined
+>Foo : Foo
+>Bar : Bar
+
+    if (typeof x === "object") {
+>typeof x === "object" : boolean
+>typeof x : string
+>x : string | Foo | Bar | undefined
+>"object" : "object"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+>f3 : (x: string | Foo | Bar | null) => void
+>x : string | Foo | Bar | null
+>Foo : Foo
+>Bar : Bar
+>null : null
+
+    if (x && typeof x !== "string") {
+>x && typeof x !== "string" : boolean | "" | null
+>x : string | Foo | Bar | null
+>typeof x !== "string" : boolean
+>typeof x : string
+>x : string | Foo | Bar
+>"string" : "string"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+>f4 : (x: string | number | Foo | Bar | null) => void
+>x : string | number | Foo | Bar | null
+>Foo : Foo
+>Bar : Bar
+>null : null
+
+    if (x && typeof x === "object") {
+>x && typeof x === "object" : boolean | "" | 0 | null
+>x : string | number | Foo | Bar | null
+>typeof x === "object" : boolean
+>typeof x : string
+>x : string | number | Foo | Bar
+>"object" : "object"
+
+        switch(x.kind) {
+>x.kind : "foo" | "bar"
+>x : Foo | Bar
+>kind : "foo" | "bar"
+
+            case 'foo':
+>'foo' : "foo"
+
+                x.name;
+>x.name : string
+>x : Foo
+>name : string
+        }
+    }
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.js b/tests/baselines/reference/discriminantsAndTypePredicates.js
new file mode 100644
index 00000000000..66cfe056ab7
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.js
@@ -0,0 +1,59 @@
+//// [discriminantsAndTypePredicates.ts]
+// Repro from #10145
+
+interface A { type: 'A' }
+interface B { type: 'B' }
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+function isB(x: A | B): x is B { return x.type === 'B'; }
+
+function foo1(x: A | B): any {
+    x;  // A | B
+    if (isA(x)) {
+        return x;  // A
+    }
+    x;  // B
+    if (isB(x)) {
+        return x;  // B
+    }
+    x;  // never
+}
+
+function foo2(x: A | B): any {
+    x;  // A | B
+    if (x.type === 'A') {
+        return x;  // A
+    }
+    x;  // B
+    if (x.type === 'B') {
+        return x;  // B
+    }
+    x;  // never
+}
+
+//// [discriminantsAndTypePredicates.js]
+// Repro from #10145
+function isA(x) { return x.type === 'A'; }
+function isB(x) { return x.type === 'B'; }
+function foo1(x) {
+    x; // A | B
+    if (isA(x)) {
+        return x; // A
+    }
+    x; // B
+    if (isB(x)) {
+        return x; // B
+    }
+    x; // never
+}
+function foo2(x) {
+    x; // A | B
+    if (x.type === 'A') {
+        return x; // A
+    }
+    x; // B
+    if (x.type === 'B') {
+        return x; // B
+    }
+    x; // never
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.symbols b/tests/baselines/reference/discriminantsAndTypePredicates.symbols
new file mode 100644
index 00000000000..4f412823936
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.symbols
@@ -0,0 +1,94 @@
+=== tests/cases/compiler/discriminantsAndTypePredicates.ts ===
+// Repro from #10145
+
+interface A { type: 'A' }
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>type : Symbol(A.type, Decl(discriminantsAndTypePredicates.ts, 2, 13))
+
+interface B { type: 'B' }
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+>isA : Symbol(isA, Decl(discriminantsAndTypePredicates.ts, 3, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 5, 13))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function isB(x: A | B): x is B { return x.type === 'B'; }
+>isB : Symbol(isB, Decl(discriminantsAndTypePredicates.ts, 5, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 6, 13))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+function foo1(x: A | B): any {
+>foo1 : Symbol(foo1, Decl(discriminantsAndTypePredicates.ts, 6, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+
+    x;  // A | B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+    if (isA(x)) {
+>isA : Symbol(isA, Decl(discriminantsAndTypePredicates.ts, 3, 25))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+        return x;  // A
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+    }
+    x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+    if (isB(x)) {
+>isB : Symbol(isB, Decl(discriminantsAndTypePredicates.ts, 5, 57))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+
+        return x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+    }
+    x;  // never
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 8, 14))
+}
+
+function foo2(x: A | B): any {
+>foo2 : Symbol(foo2, Decl(discriminantsAndTypePredicates.ts, 18, 1))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>A : Symbol(A, Decl(discriminantsAndTypePredicates.ts, 0, 0))
+>B : Symbol(B, Decl(discriminantsAndTypePredicates.ts, 2, 25))
+
+    x;  // A | B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+
+    if (x.type === 'A') {
+>x.type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>type : Symbol(type, Decl(discriminantsAndTypePredicates.ts, 2, 13), Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+        return x;  // A
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+    }
+    x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+
+    if (x.type === 'B') {
+>x.type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+>type : Symbol(B.type, Decl(discriminantsAndTypePredicates.ts, 3, 13))
+
+        return x;  // B
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+    }
+    x;  // never
+>x : Symbol(x, Decl(discriminantsAndTypePredicates.ts, 20, 14))
+}
diff --git a/tests/baselines/reference/discriminantsAndTypePredicates.types b/tests/baselines/reference/discriminantsAndTypePredicates.types
new file mode 100644
index 00000000000..f7675b14756
--- /dev/null
+++ b/tests/baselines/reference/discriminantsAndTypePredicates.types
@@ -0,0 +1,104 @@
+=== tests/cases/compiler/discriminantsAndTypePredicates.ts ===
+// Repro from #10145
+
+interface A { type: 'A' }
+>A : A
+>type : "A"
+
+interface B { type: 'B' }
+>B : B
+>type : "B"
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+>isA : (x: A | B) => x is A
+>x : A | B
+>A : A
+>B : B
+>x : any
+>A : A
+>x.type === 'A' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'A' : "A"
+
+function isB(x: A | B): x is B { return x.type === 'B'; }
+>isB : (x: A | B) => x is B
+>x : A | B
+>A : A
+>B : B
+>x : any
+>B : B
+>x.type === 'B' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'B' : "B"
+
+function foo1(x: A | B): any {
+>foo1 : (x: A | B) => any
+>x : A | B
+>A : A
+>B : B
+
+    x;  // A | B
+>x : A | B
+
+    if (isA(x)) {
+>isA(x) : boolean
+>isA : (x: A | B) => x is A
+>x : A | B
+
+        return x;  // A
+>x : A
+    }
+    x;  // B
+>x : B
+
+    if (isB(x)) {
+>isB(x) : boolean
+>isB : (x: A | B) => x is B
+>x : B
+
+        return x;  // B
+>x : B
+    }
+    x;  // never
+>x : never
+}
+
+function foo2(x: A | B): any {
+>foo2 : (x: A | B) => any
+>x : A | B
+>A : A
+>B : B
+
+    x;  // A | B
+>x : A | B
+
+    if (x.type === 'A') {
+>x.type === 'A' : boolean
+>x.type : "A" | "B"
+>x : A | B
+>type : "A" | "B"
+>'A' : "A"
+
+        return x;  // A
+>x : A
+    }
+    x;  // B
+>x : B
+
+    if (x.type === 'B') {
+>x.type === 'B' : boolean
+>x.type : "B"
+>x : B
+>type : "B"
+>'B' : "B"
+
+        return x;  // B
+>x : B
+    }
+    x;  // never
+>x : never
+}
diff --git a/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.errors.txt b/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.errors.txt
new file mode 100644
index 00000000000..d0f11d056ca
--- /dev/null
+++ b/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.errors.txt
@@ -0,0 +1,13 @@
+tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts(3,17): error TS2493: Tuple type '[any]' with length '1' cannot be assigned to tuple with length '3'.
+tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts(3,29): error TS2493: Tuple type '[any]' with length '1' cannot be assigned to tuple with length '3'.
+
+
+==== tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts (2 errors) ====
+    declare function wrapper(x: any);
+    wrapper((array: [any]) => {
+        [this.test, this.test1, this.test2] = array;  // even though there is a compiler error, we should still emit lexical capture for "this"
+                    ~~~~~~~~~~
+!!! error TS2493: Tuple type '[any]' with length '1' cannot be assigned to tuple with length '3'.
+                                ~~~~~~~~~~
+!!! error TS2493: Tuple type '[any]' with length '1' cannot be assigned to tuple with length '3'.
+    });
\ No newline at end of file
diff --git a/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.js b/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.js
new file mode 100644
index 00000000000..9dd7453ba4f
--- /dev/null
+++ b/tests/baselines/reference/emitCapturingThisInTupleDestructuring1.js
@@ -0,0 +1,11 @@
+//// [emitCapturingThisInTupleDestructuring1.ts]
+declare function wrapper(x: any);
+wrapper((array: [any]) => {
+    [this.test, this.test1, this.test2] = array;  // even though there is a compiler error, we should still emit lexical capture for "this"
+});
+
+//// [emitCapturingThisInTupleDestructuring1.js]
+var _this = this;
+wrapper(function (array) {
+    _this.test = array[0], _this.test1 = array[1], _this.test2 = array[2]; // even though there is a compiler error, we should still emit lexical capture for "this"
+});
diff --git a/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.errors.txt b/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.errors.txt
new file mode 100644
index 00000000000..25207e28c79
--- /dev/null
+++ b/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.errors.txt
@@ -0,0 +1,16 @@
+tests/cases/compiler/emitCapturingThisInTupleDestructuring2.ts(8,39): error TS2493: Tuple type '[number, number]' with length '2' cannot be assigned to tuple with length '3'.
+
+
+==== tests/cases/compiler/emitCapturingThisInTupleDestructuring2.ts (1 errors) ====
+    var array1: [number, number] = [1, 2];
+    
+    class B {
+        test: number;
+        test1: any;
+        test2: any;
+        method() {
+            () => [this.test, this.test1, this.test2] = array1; // even though there is a compiler error, we should still emit lexical capture for "this" 
+                                          ~~~~~~~~~~
+!!! error TS2493: Tuple type '[number, number]' with length '2' cannot be assigned to tuple with length '3'.
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.js b/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.js
new file mode 100644
index 00000000000..f999cfe70f0
--- /dev/null
+++ b/tests/baselines/reference/emitCapturingThisInTupleDestructuring2.js
@@ -0,0 +1,23 @@
+//// [emitCapturingThisInTupleDestructuring2.ts]
+var array1: [number, number] = [1, 2];
+
+class B {
+    test: number;
+    test1: any;
+    test2: any;
+    method() {
+        () => [this.test, this.test1, this.test2] = array1; // even though there is a compiler error, we should still emit lexical capture for "this" 
+    }
+}
+
+//// [emitCapturingThisInTupleDestructuring2.js]
+var array1 = [1, 2];
+var B = (function () {
+    function B() {
+    }
+    B.prototype.method = function () {
+        var _this = this;
+        (function () { return (_this.test = array1[0], _this.test1 = array1[1], _this.test2 = array1[2], array1); }); // even though there is a compiler error, we should still emit lexical capture for "this" 
+    };
+    return B;
+}());
diff --git a/tests/baselines/reference/emitClassDeclarationWithPropertyAssignmentInES6.js b/tests/baselines/reference/emitClassDeclarationWithPropertyAssignmentInES6.js
index 03fb45b806e..91e6c506c2b 100644
--- a/tests/baselines/reference/emitClassDeclarationWithPropertyAssignmentInES6.js
+++ b/tests/baselines/reference/emitClassDeclarationWithPropertyAssignmentInES6.js
@@ -37,8 +37,8 @@ class D {
     }
 }
 class E extends D {
-    constructor(...args) {
-        super(...args);
+    constructor() {
+        super(...arguments);
         this.z = true;
     }
 }
diff --git a/tests/baselines/reference/exportDefaultProperty.js b/tests/baselines/reference/exportDefaultProperty.js
new file mode 100644
index 00000000000..10ba6d6419a
--- /dev/null
+++ b/tests/baselines/reference/exportDefaultProperty.js
@@ -0,0 +1,76 @@
+//// [tests/cases/compiler/exportDefaultProperty.ts] ////
+
+//// [declarations.d.ts]
+// This test is just like exportEqualsProperty, but with `export default`.
+
+declare namespace foo.bar {
+    export type X = number;
+    export const X: number;
+}
+
+declare module "foobar" {
+    export default foo.bar;
+}
+
+declare module "foobarx" {
+    export default foo.bar.X;
+}
+
+//// [a.ts]
+namespace A {
+    export class B { constructor(b: number) {} }
+    export namespace B { export const b: number = 0; }
+}
+export default A.B;
+
+//// [b.ts]
+export default "foo".length;
+
+//// [index.ts]
+/// <reference path="declarations.d.ts" />
+import fooBar from "foobar";
+import X = fooBar.X;
+import X2 from "foobarx";
+const x: X = X;
+const x2: X2 = X2;
+
+import B from "./a";
+const b: B = new B(B.b);
+
+import fooLength from "./b";
+fooLength + 1;
+
+
+//// [a.js]
+"use strict";
+var A;
+(function (A) {
+    var B = (function () {
+        function B(b) {
+        }
+        return B;
+    }());
+    A.B = B;
+    var B;
+    (function (B) {
+        B.b = 0;
+    })(B = A.B || (A.B = {}));
+})(A || (A = {}));
+exports.__esModule = true;
+exports["default"] = A.B;
+//// [b.js]
+"use strict";
+exports.__esModule = true;
+exports["default"] = "foo".length;
+//// [index.js]
+"use strict";
+/// <reference path="declarations.d.ts" />
+var foobar_1 = require("foobar");
+var X = foobar_1["default"].X;
+var foobarx_1 = require("foobarx");
+var x = X;
+var x2 = foobarx_1["default"];
+var a_1 = require("./a");
+var b = new a_1["default"](a_1["default"].b);
+var b_1 = require("./b");
+b_1["default"] + 1;
diff --git a/tests/baselines/reference/exportDefaultProperty.symbols b/tests/baselines/reference/exportDefaultProperty.symbols
new file mode 100644
index 00000000000..f9edcd154cc
--- /dev/null
+++ b/tests/baselines/reference/exportDefaultProperty.symbols
@@ -0,0 +1,92 @@
+=== tests/cases/compiler/index.ts ===
+/// <reference path="declarations.d.ts" />
+import fooBar from "foobar";
+>fooBar : Symbol(fooBar, Decl(index.ts, 1, 6))
+
+import X = fooBar.X;
+>X : Symbol(X, Decl(index.ts, 1, 28))
+>fooBar : Symbol(fooBar, Decl(index.ts, 1, 6))
+>X : Symbol(fooBar.X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+
+import X2 from "foobarx";
+>X2 : Symbol(X2, Decl(index.ts, 3, 6))
+
+const x: X = X;
+>x : Symbol(x, Decl(index.ts, 4, 5))
+>X : Symbol(X, Decl(index.ts, 1, 28))
+>X : Symbol(X, Decl(index.ts, 1, 28))
+
+const x2: X2 = X2;
+>x2 : Symbol(x2, Decl(index.ts, 5, 5))
+>X2 : Symbol(X2, Decl(index.ts, 3, 6))
+>X2 : Symbol(X2, Decl(index.ts, 3, 6))
+
+import B from "./a";
+>B : Symbol(B, Decl(index.ts, 7, 6))
+
+const b: B = new B(B.b);
+>b : Symbol(b, Decl(index.ts, 8, 5))
+>B : Symbol(B, Decl(index.ts, 7, 6))
+>B : Symbol(B, Decl(index.ts, 7, 6))
+>B.b : Symbol(B.b, Decl(a.ts, 2, 37))
+>B : Symbol(B, Decl(index.ts, 7, 6))
+>b : Symbol(B.b, Decl(a.ts, 2, 37))
+
+import fooLength from "./b";
+>fooLength : Symbol(fooLength, Decl(index.ts, 10, 6))
+
+fooLength + 1;
+>fooLength : Symbol(fooLength, Decl(index.ts, 10, 6))
+
+=== tests/cases/compiler/declarations.d.ts ===
+// This test is just like exportEqualsProperty, but with `export default`.
+
+declare namespace foo.bar {
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(bar, Decl(declarations.d.ts, 2, 22))
+
+    export type X = number;
+>X : Symbol(X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+
+    export const X: number;
+>X : Symbol(X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+}
+
+declare module "foobar" {
+    export default foo.bar;
+>foo.bar : Symbol(default, Decl(declarations.d.ts, 2, 22))
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(default, Decl(declarations.d.ts, 2, 22))
+}
+
+declare module "foobarx" {
+    export default foo.bar.X;
+>foo.bar.X : Symbol(default, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+>foo.bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+>X : Symbol(default, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+}
+
+=== tests/cases/compiler/a.ts ===
+namespace A {
+>A : Symbol(A, Decl(a.ts, 0, 0))
+
+    export class B { constructor(b: number) {} }
+>B : Symbol(B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>b : Symbol(b, Decl(a.ts, 1, 33))
+
+    export namespace B { export const b: number = 0; }
+>B : Symbol(B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>b : Symbol(b, Decl(a.ts, 2, 37))
+}
+export default A.B;
+>A.B : Symbol(default, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>A : Symbol(A, Decl(a.ts, 0, 0))
+>B : Symbol(default, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+
+=== tests/cases/compiler/b.ts ===
+export default "foo".length;
+>"foo".length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+
diff --git a/tests/baselines/reference/exportDefaultProperty.types b/tests/baselines/reference/exportDefaultProperty.types
new file mode 100644
index 00000000000..47cfabfbc16
--- /dev/null
+++ b/tests/baselines/reference/exportDefaultProperty.types
@@ -0,0 +1,97 @@
+=== tests/cases/compiler/index.ts ===
+/// <reference path="declarations.d.ts" />
+import fooBar from "foobar";
+>fooBar : typeof fooBar
+
+import X = fooBar.X;
+>X : number
+>fooBar : typeof fooBar
+>X : number
+
+import X2 from "foobarx";
+>X2 : number
+
+const x: X = X;
+>x : number
+>X : number
+>X : number
+
+const x2: X2 = X2;
+>x2 : number
+>X2 : number
+>X2 : number
+
+import B from "./a";
+>B : typeof B
+
+const b: B = new B(B.b);
+>b : B
+>B : B
+>new B(B.b) : B
+>B : typeof B
+>B.b : number
+>B : typeof B
+>b : number
+
+import fooLength from "./b";
+>fooLength : number
+
+fooLength + 1;
+>fooLength + 1 : number
+>fooLength : number
+>1 : number
+
+=== tests/cases/compiler/declarations.d.ts ===
+// This test is just like exportEqualsProperty, but with `export default`.
+
+declare namespace foo.bar {
+>foo : typeof foo
+>bar : typeof bar
+
+    export type X = number;
+>X : number
+
+    export const X: number;
+>X : number
+}
+
+declare module "foobar" {
+    export default foo.bar;
+>foo.bar : typeof default
+>foo : typeof foo
+>bar : typeof default
+}
+
+declare module "foobarx" {
+    export default foo.bar.X;
+>foo.bar.X : number
+>foo.bar : typeof foo.bar
+>foo : typeof foo
+>bar : typeof foo.bar
+>X : number
+}
+
+=== tests/cases/compiler/a.ts ===
+namespace A {
+>A : typeof A
+
+    export class B { constructor(b: number) {} }
+>B : B
+>b : number
+
+    export namespace B { export const b: number = 0; }
+>B : typeof B
+>b : number
+>0 : number
+}
+export default A.B;
+>A.B : typeof default
+>A : typeof A
+>B : typeof default
+
+=== tests/cases/compiler/b.ts ===
+export default "foo".length;
+>"foo".length : number
+>"foo" : string
+>length : number
+
diff --git a/tests/baselines/reference/exportEqualsProperty.js b/tests/baselines/reference/exportEqualsProperty.js
new file mode 100644
index 00000000000..2fd8a8c8511
--- /dev/null
+++ b/tests/baselines/reference/exportEqualsProperty.js
@@ -0,0 +1,72 @@
+//// [tests/cases/compiler/exportEqualsProperty.ts] ////
+
+//// [declarations.d.ts]
+// This test is just like exportDefaultProperty, but with `export =`.
+
+declare namespace foo.bar {
+    export type X = number;
+    export const X: number;
+}
+
+declare module "foobar" {
+    export = foo.bar;
+}
+
+declare module "foobarx" {
+    export = foo.bar.X;
+}
+
+//// [a.ts]
+namespace A {
+    export class B { constructor(b: number) {} }
+    export namespace B { export const b: number = 0; }
+}
+export = A.B;
+
+//// [b.ts]
+export = "foo".length;
+
+//// [index.ts]
+/// <reference path="declarations.d.ts" />
+import { X } from "foobar";
+import X2 = require("foobarx");
+const x: X = X;
+const x2: X2 = X2;
+
+import B = require("./a");
+const b: B = new B(B.b);
+
+import fooLength = require("./b");
+fooLength + 1;
+
+
+//// [a.js]
+"use strict";
+var A;
+(function (A) {
+    var B = (function () {
+        function B(b) {
+        }
+        return B;
+    }());
+    A.B = B;
+    var B;
+    (function (B) {
+        B.b = 0;
+    })(B = A.B || (A.B = {}));
+})(A || (A = {}));
+module.exports = A.B;
+//// [b.js]
+"use strict";
+module.exports = "foo".length;
+//// [index.js]
+"use strict";
+/// <reference path="declarations.d.ts" />
+var foobar_1 = require("foobar");
+var X2 = require("foobarx");
+var x = foobar_1.X;
+var x2 = X2;
+var B = require("./a");
+var b = new B(B.b);
+var fooLength = require("./b");
+fooLength + 1;
diff --git a/tests/baselines/reference/exportEqualsProperty.symbols b/tests/baselines/reference/exportEqualsProperty.symbols
new file mode 100644
index 00000000000..43c9ed32518
--- /dev/null
+++ b/tests/baselines/reference/exportEqualsProperty.symbols
@@ -0,0 +1,87 @@
+=== tests/cases/compiler/index.ts ===
+/// <reference path="declarations.d.ts" />
+import { X } from "foobar";
+>X : Symbol(X, Decl(index.ts, 1, 8))
+
+import X2 = require("foobarx");
+>X2 : Symbol(X2, Decl(index.ts, 1, 27))
+
+const x: X = X;
+>x : Symbol(x, Decl(index.ts, 3, 5))
+>X : Symbol(X, Decl(index.ts, 1, 8))
+>X : Symbol(X, Decl(index.ts, 1, 8))
+
+const x2: X2 = X2;
+>x2 : Symbol(x2, Decl(index.ts, 4, 5))
+>X2 : Symbol(X2, Decl(index.ts, 1, 27))
+>X2 : Symbol(X2, Decl(index.ts, 1, 27))
+
+import B = require("./a");
+>B : Symbol(B, Decl(index.ts, 4, 18))
+
+const b: B = new B(B.b);
+>b : Symbol(b, Decl(index.ts, 7, 5))
+>B : Symbol(B, Decl(index.ts, 4, 18))
+>B : Symbol(B, Decl(index.ts, 4, 18))
+>B.b : Symbol(B.b, Decl(a.ts, 2, 37))
+>B : Symbol(B, Decl(index.ts, 4, 18))
+>b : Symbol(B.b, Decl(a.ts, 2, 37))
+
+import fooLength = require("./b");
+>fooLength : Symbol(fooLength, Decl(index.ts, 7, 24))
+
+fooLength + 1;
+>fooLength : Symbol(fooLength, Decl(index.ts, 7, 24))
+
+=== tests/cases/compiler/declarations.d.ts ===
+// This test is just like exportDefaultProperty, but with `export =`.
+
+declare namespace foo.bar {
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(bar, Decl(declarations.d.ts, 2, 22))
+
+    export type X = number;
+>X : Symbol(X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+
+    export const X: number;
+>X : Symbol(X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+}
+
+declare module "foobar" {
+    export = foo.bar;
+>foo.bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+}
+
+declare module "foobarx" {
+    export = foo.bar.X;
+>foo.bar.X : Symbol(foo.bar.X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+>foo.bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+>foo : Symbol(foo, Decl(declarations.d.ts, 0, 0))
+>bar : Symbol(foo.bar, Decl(declarations.d.ts, 2, 22))
+>X : Symbol(foo.bar.X, Decl(declarations.d.ts, 2, 27), Decl(declarations.d.ts, 4, 16))
+}
+
+=== tests/cases/compiler/a.ts ===
+namespace A {
+>A : Symbol(A, Decl(a.ts, 0, 0))
+
+    export class B { constructor(b: number) {} }
+>B : Symbol(B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>b : Symbol(b, Decl(a.ts, 1, 33))
+
+    export namespace B { export const b: number = 0; }
+>B : Symbol(B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>b : Symbol(b, Decl(a.ts, 2, 37))
+}
+export = A.B;
+>A.B : Symbol(A.B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+>A : Symbol(A, Decl(a.ts, 0, 0))
+>B : Symbol(A.B, Decl(a.ts, 0, 13), Decl(a.ts, 1, 48))
+
+=== tests/cases/compiler/b.ts ===
+export = "foo".length;
+>"foo".length : Symbol(String.length, Decl(lib.d.ts, --, --))
+>length : Symbol(String.length, Decl(lib.d.ts, --, --))
+
diff --git a/tests/baselines/reference/exportEqualsProperty.types b/tests/baselines/reference/exportEqualsProperty.types
new file mode 100644
index 00000000000..a92af53b12b
--- /dev/null
+++ b/tests/baselines/reference/exportEqualsProperty.types
@@ -0,0 +1,92 @@
+=== tests/cases/compiler/index.ts ===
+/// <reference path="declarations.d.ts" />
+import { X } from "foobar";
+>X : number
+
+import X2 = require("foobarx");
+>X2 : number
+
+const x: X = X;
+>x : number
+>X : number
+>X : number
+
+const x2: X2 = X2;
+>x2 : number
+>X2 : number
+>X2 : number
+
+import B = require("./a");
+>B : typeof B
+
+const b: B = new B(B.b);
+>b : B
+>B : B
+>new B(B.b) : B
+>B : typeof B
+>B.b : number
+>B : typeof B
+>b : number
+
+import fooLength = require("./b");
+>fooLength : number
+
+fooLength + 1;
+>fooLength + 1 : number
+>fooLength : number
+>1 : number
+
+=== tests/cases/compiler/declarations.d.ts ===
+// This test is just like exportDefaultProperty, but with `export =`.
+
+declare namespace foo.bar {
+>foo : typeof foo
+>bar : typeof bar
+
+    export type X = number;
+>X : number
+
+    export const X: number;
+>X : number
+}
+
+declare module "foobar" {
+    export = foo.bar;
+>foo.bar : typeof foo.bar
+>foo : typeof foo
+>bar : typeof foo.bar
+}
+
+declare module "foobarx" {
+    export = foo.bar.X;
+>foo.bar.X : number
+>foo.bar : typeof foo.bar
+>foo : typeof foo
+>bar : typeof foo.bar
+>X : number
+}
+
+=== tests/cases/compiler/a.ts ===
+namespace A {
+>A : typeof A
+
+    export class B { constructor(b: number) {} }
+>B : B
+>b : number
+
+    export namespace B { export const b: number = 0; }
+>B : typeof B
+>b : number
+>0 : number
+}
+export = A.B;
+>A.B : typeof A.B
+>A : typeof A
+>B : typeof A.B
+
+=== tests/cases/compiler/b.ts ===
+export = "foo".length;
+>"foo".length : number
+>"foo" : string
+>length : number
+
diff --git a/tests/baselines/reference/exportToString.js b/tests/baselines/reference/exportToString.js
new file mode 100644
index 00000000000..190d1693c24
--- /dev/null
+++ b/tests/baselines/reference/exportToString.js
@@ -0,0 +1,9 @@
+//// [exportToString.ts]
+const toString = 0;
+export { toString };
+
+
+//// [exportToString.js]
+"use strict";
+var toString = 0;
+exports.toString = toString;
diff --git a/tests/baselines/reference/exportToString.symbols b/tests/baselines/reference/exportToString.symbols
new file mode 100644
index 00000000000..ce5446317a4
--- /dev/null
+++ b/tests/baselines/reference/exportToString.symbols
@@ -0,0 +1,7 @@
+=== tests/cases/compiler/exportToString.ts ===
+const toString = 0;
+>toString : Symbol(toString, Decl(exportToString.ts, 0, 5))
+
+export { toString };
+>toString : Symbol(toString, Decl(exportToString.ts, 1, 8))
+
diff --git a/tests/baselines/reference/exportToString.types b/tests/baselines/reference/exportToString.types
new file mode 100644
index 00000000000..17037852f3b
--- /dev/null
+++ b/tests/baselines/reference/exportToString.types
@@ -0,0 +1,8 @@
+=== tests/cases/compiler/exportToString.ts ===
+const toString = 0;
+>toString : number
+>0 : number
+
+export { toString };
+>toString : number
+
diff --git a/tests/baselines/reference/implicitConstParameters.errors.txt b/tests/baselines/reference/implicitConstParameters.errors.txt
new file mode 100644
index 00000000000..95ff60d71f8
--- /dev/null
+++ b/tests/baselines/reference/implicitConstParameters.errors.txt
@@ -0,0 +1,65 @@
+tests/cases/compiler/implicitConstParameters.ts(39,27): error TS2532: Object is possibly 'undefined'.
+tests/cases/compiler/implicitConstParameters.ts(45,27): error TS2532: Object is possibly 'undefined'.
+
+
+==== tests/cases/compiler/implicitConstParameters.ts (2 errors) ====
+    
+    function doSomething(cb: () => void) {
+        cb();
+    }
+    
+    function fn(x: number | string) {
+      if (typeof x === 'number') {
+          doSomething(() => x.toFixed());
+      }
+    }
+    
+    function f1(x: string | undefined) {
+        if (!x) {
+            return;
+        }
+        doSomething(() => x.length);
+    }
+    
+    function f2(x: string | undefined) {
+        if (x) {
+            doSomething(() => {
+                doSomething(() => x.length);
+            });
+        }
+    }
+    
+    function f3(x: string | undefined) {
+        inner();
+        function inner() {
+            if (x) {
+                doSomething(() => x.length);
+            }
+        }
+    }
+    
+    function f4(x: string | undefined) {
+        x = "abc";  // causes x to be considered non-const
+        if (x) {
+            doSomething(() => x.length);
+                              ~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+    }
+    
+    function f5(x: string | undefined) {
+        if (x) {
+            doSomething(() => x.length);
+                              ~
+!!! error TS2532: Object is possibly 'undefined'.
+        }
+        x = "abc";  // causes x to be considered non-const
+    }
+    
+    
+    function f6(x: string | undefined) {
+        const y = x || "";
+        if (x) {
+            doSomething(() => y.length);
+        }
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/implicitConstParameters.js b/tests/baselines/reference/implicitConstParameters.js
new file mode 100644
index 00000000000..a5faf5b1253
--- /dev/null
+++ b/tests/baselines/reference/implicitConstParameters.js
@@ -0,0 +1,106 @@
+//// [implicitConstParameters.ts]
+
+function doSomething(cb: () => void) {
+    cb();
+}
+
+function fn(x: number | string) {
+  if (typeof x === 'number') {
+      doSomething(() => x.toFixed());
+  }
+}
+
+function f1(x: string | undefined) {
+    if (!x) {
+        return;
+    }
+    doSomething(() => x.length);
+}
+
+function f2(x: string | undefined) {
+    if (x) {
+        doSomething(() => {
+            doSomething(() => x.length);
+        });
+    }
+}
+
+function f3(x: string | undefined) {
+    inner();
+    function inner() {
+        if (x) {
+            doSomething(() => x.length);
+        }
+    }
+}
+
+function f4(x: string | undefined) {
+    x = "abc";  // causes x to be considered non-const
+    if (x) {
+        doSomething(() => x.length);
+    }
+}
+
+function f5(x: string | undefined) {
+    if (x) {
+        doSomething(() => x.length);
+    }
+    x = "abc";  // causes x to be considered non-const
+}
+
+
+function f6(x: string | undefined) {
+    const y = x || "";
+    if (x) {
+        doSomething(() => y.length);
+    }
+}
+
+//// [implicitConstParameters.js]
+function doSomething(cb) {
+    cb();
+}
+function fn(x) {
+    if (typeof x === 'number') {
+        doSomething(function () { return x.toFixed(); });
+    }
+}
+function f1(x) {
+    if (!x) {
+        return;
+    }
+    doSomething(function () { return x.length; });
+}
+function f2(x) {
+    if (x) {
+        doSomething(function () {
+            doSomething(function () { return x.length; });
+        });
+    }
+}
+function f3(x) {
+    inner();
+    function inner() {
+        if (x) {
+            doSomething(function () { return x.length; });
+        }
+    }
+}
+function f4(x) {
+    x = "abc"; // causes x to be considered non-const
+    if (x) {
+        doSomething(function () { return x.length; });
+    }
+}
+function f5(x) {
+    if (x) {
+        doSomething(function () { return x.length; });
+    }
+    x = "abc"; // causes x to be considered non-const
+}
+function f6(x) {
+    var y = x || "";
+    if (x) {
+        doSomething(function () { return y.length; });
+    }
+}
diff --git a/tests/baselines/reference/indexWithUndefinedAndNull.js b/tests/baselines/reference/indexWithUndefinedAndNull.js
new file mode 100644
index 00000000000..f885323efae
--- /dev/null
+++ b/tests/baselines/reference/indexWithUndefinedAndNull.js
@@ -0,0 +1,22 @@
+//// [indexWithUndefinedAndNull.ts]
+interface N {
+    [n: number]: string;
+}
+interface S {
+    [s: string]: number;
+}
+let n: N;
+let s: S;
+let str: string = n[undefined];
+str = n[null];
+let num: number = s[undefined];
+num = s[null];
+
+
+//// [indexWithUndefinedAndNull.js]
+var n;
+var s;
+var str = n[undefined];
+str = n[null];
+var num = s[undefined];
+num = s[null];
diff --git a/tests/baselines/reference/indexWithUndefinedAndNull.symbols b/tests/baselines/reference/indexWithUndefinedAndNull.symbols
new file mode 100644
index 00000000000..15e8bbdbf41
--- /dev/null
+++ b/tests/baselines/reference/indexWithUndefinedAndNull.symbols
@@ -0,0 +1,39 @@
+=== tests/cases/compiler/indexWithUndefinedAndNull.ts ===
+interface N {
+>N : Symbol(N, Decl(indexWithUndefinedAndNull.ts, 0, 0))
+
+    [n: number]: string;
+>n : Symbol(n, Decl(indexWithUndefinedAndNull.ts, 1, 5))
+}
+interface S {
+>S : Symbol(S, Decl(indexWithUndefinedAndNull.ts, 2, 1))
+
+    [s: string]: number;
+>s : Symbol(s, Decl(indexWithUndefinedAndNull.ts, 4, 5))
+}
+let n: N;
+>n : Symbol(n, Decl(indexWithUndefinedAndNull.ts, 6, 3))
+>N : Symbol(N, Decl(indexWithUndefinedAndNull.ts, 0, 0))
+
+let s: S;
+>s : Symbol(s, Decl(indexWithUndefinedAndNull.ts, 7, 3))
+>S : Symbol(S, Decl(indexWithUndefinedAndNull.ts, 2, 1))
+
+let str: string = n[undefined];
+>str : Symbol(str, Decl(indexWithUndefinedAndNull.ts, 8, 3))
+>n : Symbol(n, Decl(indexWithUndefinedAndNull.ts, 6, 3))
+>undefined : Symbol(undefined)
+
+str = n[null];
+>str : Symbol(str, Decl(indexWithUndefinedAndNull.ts, 8, 3))
+>n : Symbol(n, Decl(indexWithUndefinedAndNull.ts, 6, 3))
+
+let num: number = s[undefined];
+>num : Symbol(num, Decl(indexWithUndefinedAndNull.ts, 10, 3))
+>s : Symbol(s, Decl(indexWithUndefinedAndNull.ts, 7, 3))
+>undefined : Symbol(undefined)
+
+num = s[null];
+>num : Symbol(num, Decl(indexWithUndefinedAndNull.ts, 10, 3))
+>s : Symbol(s, Decl(indexWithUndefinedAndNull.ts, 7, 3))
+
diff --git a/tests/baselines/reference/indexWithUndefinedAndNull.types b/tests/baselines/reference/indexWithUndefinedAndNull.types
new file mode 100644
index 00000000000..07b6050503a
--- /dev/null
+++ b/tests/baselines/reference/indexWithUndefinedAndNull.types
@@ -0,0 +1,47 @@
+=== tests/cases/compiler/indexWithUndefinedAndNull.ts ===
+interface N {
+>N : N
+
+    [n: number]: string;
+>n : number
+}
+interface S {
+>S : S
+
+    [s: string]: number;
+>s : string
+}
+let n: N;
+>n : N
+>N : N
+
+let s: S;
+>s : S
+>S : S
+
+let str: string = n[undefined];
+>str : string
+>n[undefined] : string
+>n : N
+>undefined : undefined
+
+str = n[null];
+>str = n[null] : string
+>str : string
+>n[null] : string
+>n : N
+>null : null
+
+let num: number = s[undefined];
+>num : number
+>s[undefined] : number
+>s : S
+>undefined : undefined
+
+num = s[null];
+>num = s[null] : number
+>num : number
+>s[null] : number
+>s : S
+>null : null
+
diff --git a/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.errors.txt b/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.errors.txt
new file mode 100644
index 00000000000..a1fce75c54e
--- /dev/null
+++ b/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.errors.txt
@@ -0,0 +1,40 @@
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(9,19): error TS2454: Variable 'n' is used before being assigned.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(9,19): error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(10,7): error TS2454: Variable 'n' is used before being assigned.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(10,7): error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(11,19): error TS2454: Variable 's' is used before being assigned.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(11,19): error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(12,7): error TS2454: Variable 's' is used before being assigned.
+tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts(12,7): error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+
+
+==== tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts (8 errors) ====
+    interface N {
+        [n: number]: string;
+    }
+    interface S {
+        [s: string]: number;
+    }
+    let n: N;
+    let s: S;
+    let str: string = n[undefined];
+                      ~
+!!! error TS2454: Variable 'n' is used before being assigned.
+                      ~~~~~~~~~~~~
+!!! error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+    str = n[null];
+          ~
+!!! error TS2454: Variable 'n' is used before being assigned.
+          ~~~~~~~
+!!! error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+    let num: number = s[undefined];
+                      ~
+!!! error TS2454: Variable 's' is used before being assigned.
+                      ~~~~~~~~~~~~
+!!! error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+    num = s[null];
+          ~
+!!! error TS2454: Variable 's' is used before being assigned.
+          ~~~~~~~
+!!! error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.js b/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.js
new file mode 100644
index 00000000000..9300b7c0549
--- /dev/null
+++ b/tests/baselines/reference/indexWithUndefinedAndNullStrictNullChecks.js
@@ -0,0 +1,22 @@
+//// [indexWithUndefinedAndNullStrictNullChecks.ts]
+interface N {
+    [n: number]: string;
+}
+interface S {
+    [s: string]: number;
+}
+let n: N;
+let s: S;
+let str: string = n[undefined];
+str = n[null];
+let num: number = s[undefined];
+num = s[null];
+
+
+//// [indexWithUndefinedAndNullStrictNullChecks.js]
+var n;
+var s;
+var str = n[undefined];
+str = n[null];
+var num = s[undefined];
+num = s[null];
diff --git a/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.js b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.js
new file mode 100644
index 00000000000..97be2137722
--- /dev/null
+++ b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.js
@@ -0,0 +1,172 @@
+//// [instanceofWithStructurallyIdenticalTypes.ts]
+// Repro from #7271
+
+class C1 { item: string }
+class C2 { item: string[] }
+class C3 { item: string }
+
+function foo1(x: C1 | C2 | C3): string {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+
+function foo2(x: C1 | C2 | C3): string {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+
+// More tests
+
+class A { a: string }
+class A1 extends A { }
+class A2 { a: string }
+class B extends A { b: string }
+
+function goo(x: A) {
+    if (x instanceof A) {
+        x;  // A
+    }
+    else {
+        x;  // never
+    }
+    if (x instanceof A1) {
+        x;  // A1
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof A2) {
+        x;  // A2
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof B) {
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+}
+
+
+//// [instanceofWithStructurallyIdenticalTypes.js]
+// Repro from #7271
+var __extends = (this && this.__extends) || function (d, b) {
+    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
+    function __() { this.constructor = d; }
+    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+};
+var C1 = (function () {
+    function C1() {
+    }
+    return C1;
+}());
+var C2 = (function () {
+    function C2() {
+    }
+    return C2;
+}());
+var C3 = (function () {
+    function C3() {
+    }
+    return C3;
+}());
+function foo1(x) {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+function isC1(c) { return c instanceof C1; }
+function isC2(c) { return c instanceof C2; }
+function isC3(c) { return c instanceof C3; }
+function foo2(x) {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+// More tests
+var A = (function () {
+    function A() {
+    }
+    return A;
+}());
+var A1 = (function (_super) {
+    __extends(A1, _super);
+    function A1() {
+        _super.apply(this, arguments);
+    }
+    return A1;
+}(A));
+var A2 = (function () {
+    function A2() {
+    }
+    return A2;
+}());
+var B = (function (_super) {
+    __extends(B, _super);
+    function B() {
+        _super.apply(this, arguments);
+    }
+    return B;
+}(A));
+function goo(x) {
+    if (x instanceof A) {
+        x; // A
+    }
+    else {
+        x; // never
+    }
+    if (x instanceof A1) {
+        x; // A1
+    }
+    else {
+        x; // A
+    }
+    if (x instanceof A2) {
+        x; // A2
+    }
+    else {
+        x; // A
+    }
+    if (x instanceof B) {
+        x; // B
+    }
+    else {
+        x; // A
+    }
+}
diff --git a/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.symbols b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.symbols
new file mode 100644
index 00000000000..f2eb40eea47
--- /dev/null
+++ b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.symbols
@@ -0,0 +1,192 @@
+=== tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts ===
+// Repro from #7271
+
+class C1 { item: string }
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
+
+class C2 { item: string[] }
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>item : Symbol(C2.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 10))
+
+class C3 { item: string }
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+>item : Symbol(C3.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+
+function foo1(x: C1 | C2 | C3): string {
+>foo1 : Symbol(foo1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 25))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+
+    if (x instanceof C1) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+
+        return x.item;
+>x.item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
+    }
+    else if (x instanceof C2) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+
+        return x.item[0];
+>x.item : Symbol(C2.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>item : Symbol(C2.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 10))
+    }
+    else if (x instanceof C3) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+
+        return x.item;
+>x.item : Symbol(C3.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 6, 14))
+>item : Symbol(C3.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+    }
+    return "error";
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+>isC1 : Symbol(isC1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 17, 1))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+>isC2 : Symbol(isC2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 66))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 20, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 20, 14))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 20, 14))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+>isC3 : Symbol(isC3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 20, 66))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 21, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 21, 14))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+>c : Symbol(c, Decl(instanceofWithStructurallyIdenticalTypes.ts, 21, 14))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+
+function foo2(x: C1 | C2 | C3): string {
+>foo2 : Symbol(foo2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 21, 66))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+>C1 : Symbol(C1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 0, 0))
+>C2 : Symbol(C2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 25))
+>C3 : Symbol(C3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 27))
+
+    if (isC1(x)) {
+>isC1 : Symbol(isC1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 17, 1))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+
+        return x.item;
+>x.item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+>item : Symbol(C1.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 2, 10))
+    }
+    else if (isC2(x)) {
+>isC2 : Symbol(isC2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 19, 66))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+
+        return x.item[0];
+>x.item : Symbol(C2.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+>item : Symbol(C2.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 3, 10))
+    }
+    else if (isC3(x)) {
+>isC3 : Symbol(isC3, Decl(instanceofWithStructurallyIdenticalTypes.ts, 20, 66))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+
+        return x.item;
+>x.item : Symbol(C3.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 23, 14))
+>item : Symbol(C3.item, Decl(instanceofWithStructurallyIdenticalTypes.ts, 4, 10))
+    }
+    return "error";
+}
+
+// More tests
+
+class A { a: string }
+>A : Symbol(A, Decl(instanceofWithStructurallyIdenticalTypes.ts, 34, 1))
+>a : Symbol(A.a, Decl(instanceofWithStructurallyIdenticalTypes.ts, 38, 9))
+
+class A1 extends A { }
+>A1 : Symbol(A1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 38, 21))
+>A : Symbol(A, Decl(instanceofWithStructurallyIdenticalTypes.ts, 34, 1))
+
+class A2 { a: string }
+>A2 : Symbol(A2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 39, 22))
+>a : Symbol(A2.a, Decl(instanceofWithStructurallyIdenticalTypes.ts, 40, 10))
+
+class B extends A { b: string }
+>B : Symbol(B, Decl(instanceofWithStructurallyIdenticalTypes.ts, 40, 22))
+>A : Symbol(A, Decl(instanceofWithStructurallyIdenticalTypes.ts, 34, 1))
+>b : Symbol(B.b, Decl(instanceofWithStructurallyIdenticalTypes.ts, 41, 19))
+
+function goo(x: A) {
+>goo : Symbol(goo, Decl(instanceofWithStructurallyIdenticalTypes.ts, 41, 31))
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+>A : Symbol(A, Decl(instanceofWithStructurallyIdenticalTypes.ts, 34, 1))
+
+    if (x instanceof A) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+>A : Symbol(A, Decl(instanceofWithStructurallyIdenticalTypes.ts, 34, 1))
+
+        x;  // A
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    else {
+        x;  // never
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    if (x instanceof A1) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+>A1 : Symbol(A1, Decl(instanceofWithStructurallyIdenticalTypes.ts, 38, 21))
+
+        x;  // A1
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    else {
+        x;  // A
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    if (x instanceof A2) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+>A2 : Symbol(A2, Decl(instanceofWithStructurallyIdenticalTypes.ts, 39, 22))
+
+        x;  // A2
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    else {
+        x;  // A
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    if (x instanceof B) {
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+>B : Symbol(B, Decl(instanceofWithStructurallyIdenticalTypes.ts, 40, 22))
+
+        x;  // B
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+    else {
+        x;  // A
+>x : Symbol(x, Decl(instanceofWithStructurallyIdenticalTypes.ts, 43, 13))
+    }
+}
+
diff --git a/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.types b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.types
new file mode 100644
index 00000000000..d98a6025796
--- /dev/null
+++ b/tests/baselines/reference/instanceofWithStructurallyIdenticalTypes.types
@@ -0,0 +1,211 @@
+=== tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts ===
+// Repro from #7271
+
+class C1 { item: string }
+>C1 : C1
+>item : string
+
+class C2 { item: string[] }
+>C2 : C2
+>item : string[]
+
+class C3 { item: string }
+>C3 : C3
+>item : string
+
+function foo1(x: C1 | C2 | C3): string {
+>foo1 : (x: C1 | C2 | C3) => string
+>x : C1 | C2 | C3
+>C1 : C1
+>C2 : C2
+>C3 : C3
+
+    if (x instanceof C1) {
+>x instanceof C1 : boolean
+>x : C1 | C2 | C3
+>C1 : typeof C1
+
+        return x.item;
+>x.item : string
+>x : C1
+>item : string
+    }
+    else if (x instanceof C2) {
+>x instanceof C2 : boolean
+>x : C2 | C3
+>C2 : typeof C2
+
+        return x.item[0];
+>x.item[0] : string
+>x.item : string[]
+>x : C2
+>item : string[]
+>0 : number
+    }
+    else if (x instanceof C3) {
+>x instanceof C3 : boolean
+>x : C3
+>C3 : typeof C3
+
+        return x.item;
+>x.item : string
+>x : C3
+>item : string
+    }
+    return "error";
+>"error" : string
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+>isC1 : (c: C1 | C2 | C3) => c is C1
+>c : C1 | C2 | C3
+>C1 : C1
+>C2 : C2
+>C3 : C3
+>c : any
+>C1 : C1
+>c instanceof C1 : boolean
+>c : C1 | C2 | C3
+>C1 : typeof C1
+
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+>isC2 : (c: C1 | C2 | C3) => c is C2
+>c : C1 | C2 | C3
+>C1 : C1
+>C2 : C2
+>C3 : C3
+>c : any
+>C2 : C2
+>c instanceof C2 : boolean
+>c : C1 | C2 | C3
+>C2 : typeof C2
+
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+>isC3 : (c: C1 | C2 | C3) => c is C3
+>c : C1 | C2 | C3
+>C1 : C1
+>C2 : C2
+>C3 : C3
+>c : any
+>C3 : C3
+>c instanceof C3 : boolean
+>c : C1 | C2 | C3
+>C3 : typeof C3
+
+function foo2(x: C1 | C2 | C3): string {
+>foo2 : (x: C1 | C2 | C3) => string
+>x : C1 | C2 | C3
+>C1 : C1
+>C2 : C2
+>C3 : C3
+
+    if (isC1(x)) {
+>isC1(x) : boolean
+>isC1 : (c: C1 | C2 | C3) => c is C1
+>x : C1 | C2 | C3
+
+        return x.item;
+>x.item : string
+>x : C1
+>item : string
+    }
+    else if (isC2(x)) {
+>isC2(x) : boolean
+>isC2 : (c: C1 | C2 | C3) => c is C2
+>x : C2 | C3
+
+        return x.item[0];
+>x.item[0] : string
+>x.item : string[]
+>x : C2
+>item : string[]
+>0 : number
+    }
+    else if (isC3(x)) {
+>isC3(x) : boolean
+>isC3 : (c: C1 | C2 | C3) => c is C3
+>x : C3
+
+        return x.item;
+>x.item : string
+>x : C3
+>item : string
+    }
+    return "error";
+>"error" : string
+}
+
+// More tests
+
+class A { a: string }
+>A : A
+>a : string
+
+class A1 extends A { }
+>A1 : A1
+>A : A
+
+class A2 { a: string }
+>A2 : A2
+>a : string
+
+class B extends A { b: string }
+>B : B
+>A : A
+>b : string
+
+function goo(x: A) {
+>goo : (x: A) => void
+>x : A
+>A : A
+
+    if (x instanceof A) {
+>x instanceof A : boolean
+>x : A
+>A : typeof A
+
+        x;  // A
+>x : A
+    }
+    else {
+        x;  // never
+>x : never
+    }
+    if (x instanceof A1) {
+>x instanceof A1 : boolean
+>x : A
+>A1 : typeof A1
+
+        x;  // A1
+>x : A1
+    }
+    else {
+        x;  // A
+>x : A
+    }
+    if (x instanceof A2) {
+>x instanceof A2 : boolean
+>x : A
+>A2 : typeof A2
+
+        x;  // A2
+>x : A2
+    }
+    else {
+        x;  // A
+>x : A
+    }
+    if (x instanceof B) {
+>x instanceof B : boolean
+>x : A
+>B : typeof B
+
+        x;  // B
+>x : B
+    }
+    else {
+        x;  // A
+>x : A
+    }
+}
+
diff --git a/tests/baselines/reference/library-reference-13.trace.json b/tests/baselines/reference/library-reference-13.trace.json
index d8dfb57c2a6..a23f0ef0ca5 100644
--- a/tests/baselines/reference/library-reference-13.trace.json
+++ b/tests/baselines/reference/library-reference-13.trace.json
@@ -1,5 +1,5 @@
 [
-    "======== Resolving type reference directive 'jquery', containing file '/a/b/__inferred type names__.ts', root directory '/a/types'. ========",
+    "======== Resolving type reference directive 'jquery', containing file '/__inferred type names__.ts', root directory '/a/types'. ========",
     "Resolving with primary search path '/a/types'",
     "File '/a/types/jquery/package.json' does not exist.",
     "File '/a/types/jquery/index.d.ts' exist - use it as a name resolution result.",
diff --git a/tests/baselines/reference/library-reference-14.trace.json b/tests/baselines/reference/library-reference-14.trace.json
index d8dfb57c2a6..fb3a2bb7da4 100644
--- a/tests/baselines/reference/library-reference-14.trace.json
+++ b/tests/baselines/reference/library-reference-14.trace.json
@@ -1,5 +1,5 @@
 [
-    "======== Resolving type reference directive 'jquery', containing file '/a/b/__inferred type names__.ts', root directory '/a/types'. ========",
+    "======== Resolving type reference directive 'jquery', containing file '/a/__inferred type names__.ts', root directory '/a/types'. ========",
     "Resolving with primary search path '/a/types'",
     "File '/a/types/jquery/package.json' does not exist.",
     "File '/a/types/jquery/index.d.ts' exist - use it as a name resolution result.",
diff --git a/tests/baselines/reference/library-reference-15.trace.json b/tests/baselines/reference/library-reference-15.trace.json
index 3e9d7dba1d2..e23517976b0 100644
--- a/tests/baselines/reference/library-reference-15.trace.json
+++ b/tests/baselines/reference/library-reference-15.trace.json
@@ -1,5 +1,5 @@
 [
-    "======== Resolving type reference directive 'jquery', containing file '/a/b/__inferred type names__.ts', root directory 'types'. ========",
+    "======== Resolving type reference directive 'jquery', containing file '/a/__inferred type names__.ts', root directory 'types'. ========",
     "Resolving with primary search path 'types'",
     "File 'types/jquery/package.json' does not exist.",
     "File 'types/jquery/index.d.ts' exist - use it as a name resolution result.",
diff --git a/tests/baselines/reference/library-reference-2.trace.json b/tests/baselines/reference/library-reference-2.trace.json
index c26f7d2763d..64cdd809183 100644
--- a/tests/baselines/reference/library-reference-2.trace.json
+++ b/tests/baselines/reference/library-reference-2.trace.json
@@ -5,7 +5,7 @@
     "'package.json' has 'types' field 'jquery.d.ts' that references '/types/jquery/jquery.d.ts'.",
     "File '/types/jquery/jquery.d.ts' exist - use it as a name resolution result.",
     "======== Type reference directive 'jquery' was successfully resolved to '/types/jquery/jquery.d.ts', primary: true. ========",
-    "======== Resolving type reference directive 'jquery', containing file '/__inferred type names__.ts', root directory '/types'. ========",
+    "======== Resolving type reference directive 'jquery', containing file 'test/__inferred type names__.ts', root directory '/types'. ========",
     "Resolving with primary search path '/types'",
     "Found 'package.json' at '/types/jquery/package.json'.",
     "'package.json' has 'types' field 'jquery.d.ts' that references '/types/jquery/jquery.d.ts'.",
diff --git a/tests/baselines/reference/library-reference-5.errors.txt b/tests/baselines/reference/library-reference-5.errors.txt
index a3729bc3a99..c23b49c61de 100644
--- a/tests/baselines/reference/library-reference-5.errors.txt
+++ b/tests/baselines/reference/library-reference-5.errors.txt
@@ -1,4 +1,4 @@
-/node_modules/bar/index.d.ts(1,1): message TS4090: Conflicting library definitions for 'alpha' found at '/node_modules/bar/node_modules/alpha/index.d.ts' and '/node_modules/foo/node_modules/alpha/index.d.ts'. Copy the correct file to the 'typings' folder to resolve this conflict.
+/node_modules/bar/index.d.ts(1,1): message TS4090: Conflicting definitions for 'alpha' found at '/node_modules/bar/node_modules/alpha/index.d.ts' and '/node_modules/foo/node_modules/alpha/index.d.ts'. Consider installing a specific version of this library to resolve the conflict.
 
 
 ==== /src/root.ts (0 errors) ====
@@ -18,7 +18,7 @@
 ==== /node_modules/bar/index.d.ts (1 errors) ====
     /// <reference types="alpha" />
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! message TS4090: Conflicting library definitions for 'alpha' found at '/node_modules/bar/node_modules/alpha/index.d.ts' and '/node_modules/foo/node_modules/alpha/index.d.ts'. Copy the correct file to the 'typings' folder to resolve this conflict.
+!!! message TS4090: Conflicting definitions for 'alpha' found at '/node_modules/bar/node_modules/alpha/index.d.ts' and '/node_modules/foo/node_modules/alpha/index.d.ts'. Consider installing a specific version of this library to resolve the conflict.
     declare var bar: any;
     
 ==== /node_modules/bar/node_modules/alpha/index.d.ts (0 errors) ====
diff --git a/tests/baselines/reference/library-reference-6.trace.json b/tests/baselines/reference/library-reference-6.trace.json
index 48fb49e6c7f..fd83c1431b4 100644
--- a/tests/baselines/reference/library-reference-6.trace.json
+++ b/tests/baselines/reference/library-reference-6.trace.json
@@ -4,7 +4,7 @@
     "File '/node_modules/@types/alpha/package.json' does not exist.",
     "File '/node_modules/@types/alpha/index.d.ts' exist - use it as a name resolution result.",
     "======== Type reference directive 'alpha' was successfully resolved to '/node_modules/@types/alpha/index.d.ts', primary: true. ========",
-    "======== Resolving type reference directive 'alpha', containing file '/src/__inferred type names__.ts', root directory '/node_modules/@types'. ========",
+    "======== Resolving type reference directive 'alpha', containing file '/__inferred type names__.ts', root directory '/node_modules/@types'. ========",
     "Resolving with primary search path '/node_modules/@types'",
     "File '/node_modules/@types/alpha/package.json' does not exist.",
     "File '/node_modules/@types/alpha/index.d.ts' exist - use it as a name resolution result.",
diff --git a/tests/baselines/reference/narrowExceptionVariableInCatchClause.errors.txt b/tests/baselines/reference/narrowExceptionVariableInCatchClause.errors.txt
new file mode 100644
index 00000000000..e435b98050d
--- /dev/null
+++ b/tests/baselines/reference/narrowExceptionVariableInCatchClause.errors.txt
@@ -0,0 +1,33 @@
+tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts(11,17): error TS2339: Property 'doPanic' does not exist on type '{ type: "foo"; dontPanic(): any; }'.
+tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts(16,17): error TS2339: Property 'massage' does not exist on type 'Error'.
+
+
+==== tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts (2 errors) ====
+    declare function isFooError(x: any): x is { type: 'foo'; dontPanic(); };
+    
+    function tryCatch() {
+        try {
+            // do stuff...
+        }
+        catch (err) { // err is implicitly 'any' and cannot be annotated
+    
+            if (isFooError(err)) {
+                err.dontPanic(); // OK
+                err.doPanic(); // ERROR: Property 'doPanic' does not exist on type '{...}'
+                    ~~~~~~~
+!!! error TS2339: Property 'doPanic' does not exist on type '{ type: "foo"; dontPanic(): any; }'.
+            }
+    
+            else if (err instanceof Error) {
+                err.message;
+                err.massage; // ERROR: Property 'massage' does not exist on type 'Error'
+                    ~~~~~~~
+!!! error TS2339: Property 'massage' does not exist on type 'Error'.
+            }
+    
+            else {
+                throw err;
+            }
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/narrowExceptionVariableInCatchClause.js b/tests/baselines/reference/narrowExceptionVariableInCatchClause.js
new file mode 100644
index 00000000000..5808ed76826
--- /dev/null
+++ b/tests/baselines/reference/narrowExceptionVariableInCatchClause.js
@@ -0,0 +1,44 @@
+//// [narrowExceptionVariableInCatchClause.ts]
+declare function isFooError(x: any): x is { type: 'foo'; dontPanic(); };
+
+function tryCatch() {
+    try {
+        // do stuff...
+    }
+    catch (err) { // err is implicitly 'any' and cannot be annotated
+
+        if (isFooError(err)) {
+            err.dontPanic(); // OK
+            err.doPanic(); // ERROR: Property 'doPanic' does not exist on type '{...}'
+        }
+
+        else if (err instanceof Error) {
+            err.message;
+            err.massage; // ERROR: Property 'massage' does not exist on type 'Error'
+        }
+
+        else {
+            throw err;
+        }
+    }
+}
+
+
+//// [narrowExceptionVariableInCatchClause.js]
+function tryCatch() {
+    try {
+    }
+    catch (err) {
+        if (isFooError(err)) {
+            err.dontPanic(); // OK
+            err.doPanic(); // ERROR: Property 'doPanic' does not exist on type '{...}'
+        }
+        else if (err instanceof Error) {
+            err.message;
+            err.massage; // ERROR: Property 'massage' does not exist on type 'Error'
+        }
+        else {
+            throw err;
+        }
+    }
+}
diff --git a/tests/baselines/reference/narrowFromAnyWithInstanceof.errors.txt b/tests/baselines/reference/narrowFromAnyWithInstanceof.errors.txt
new file mode 100644
index 00000000000..3e152b0faf4
--- /dev/null
+++ b/tests/baselines/reference/narrowFromAnyWithInstanceof.errors.txt
@@ -0,0 +1,33 @@
+tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts(17,7): error TS2339: Property 'mesage' does not exist on type 'Error'.
+tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts(22,7): error TS2339: Property 'getHuors' does not exist on type 'Date'.
+
+
+==== tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts (2 errors) ====
+    declare var x: any;
+    
+    if (x instanceof Function) { // 'any' is not narrowed when target type is 'Function'
+        x();
+        x(1, 2, 3);
+        x("hello!");
+        x.prop;
+    }
+    
+    if (x instanceof Object) { // 'any' is not narrowed when target type is 'Object'
+        x.method();
+        x();
+    }
+    
+    if (x instanceof Error) { // 'any' is narrowed to types other than 'Function'/'Object'
+        x.message;
+        x.mesage;
+          ~~~~~~
+!!! error TS2339: Property 'mesage' does not exist on type 'Error'.
+    }
+    
+    if (x instanceof Date) {
+        x.getDate();
+        x.getHuors();
+          ~~~~~~~~
+!!! error TS2339: Property 'getHuors' does not exist on type 'Date'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/narrowFromAnyWithInstanceof.js b/tests/baselines/reference/narrowFromAnyWithInstanceof.js
new file mode 100644
index 00000000000..4cf1ca174aa
--- /dev/null
+++ b/tests/baselines/reference/narrowFromAnyWithInstanceof.js
@@ -0,0 +1,45 @@
+//// [narrowFromAnyWithInstanceof.ts]
+declare var x: any;
+
+if (x instanceof Function) { // 'any' is not narrowed when target type is 'Function'
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+
+if (x instanceof Object) { // 'any' is not narrowed when target type is 'Object'
+    x.method();
+    x();
+}
+
+if (x instanceof Error) { // 'any' is narrowed to types other than 'Function'/'Object'
+    x.message;
+    x.mesage;
+}
+
+if (x instanceof Date) {
+    x.getDate();
+    x.getHuors();
+}
+
+
+//// [narrowFromAnyWithInstanceof.js]
+if (x instanceof Function) {
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+if (x instanceof Object) {
+    x.method();
+    x();
+}
+if (x instanceof Error) {
+    x.message;
+    x.mesage;
+}
+if (x instanceof Date) {
+    x.getDate();
+    x.getHuors();
+}
diff --git a/tests/baselines/reference/narrowFromAnyWithTypePredicate.errors.txt b/tests/baselines/reference/narrowFromAnyWithTypePredicate.errors.txt
new file mode 100644
index 00000000000..94f41becdad
--- /dev/null
+++ b/tests/baselines/reference/narrowFromAnyWithTypePredicate.errors.txt
@@ -0,0 +1,50 @@
+tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts(22,7): error TS2339: Property 'method' does not exist on type '{}'.
+tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts(23,5): error TS2349: Cannot invoke an expression whose type lacks a call signature.
+tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts(28,7): error TS2339: Property 'mesage' does not exist on type 'Error'.
+tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts(33,7): error TS2339: Property 'getHuors' does not exist on type 'Date'.
+
+
+==== tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts (4 errors) ====
+    declare var x: any;
+    declare function isFunction(x): x is Function;
+    declare function isObject(x): x is Object;
+    declare function isAnything(x): x is {};
+    declare function isError(x): x is Error;
+    declare function isDate(x): x is Date;
+    
+    
+    if (isFunction(x)) { // 'any' is not narrowed when target type is 'Function'
+        x();
+        x(1, 2, 3);
+        x("hello!");
+        x.prop;
+    }
+    
+    if (isObject(x)) { // 'any' is not narrowed when target type is 'Object'
+        x.method();
+        x();
+    }
+    
+    if (isAnything(x)) { // 'any' is narrowed to types other than 'Function'/'Object' (including {})
+        x.method();
+          ~~~~~~
+!!! error TS2339: Property 'method' does not exist on type '{}'.
+        x();
+        ~~~
+!!! error TS2349: Cannot invoke an expression whose type lacks a call signature.
+    }
+    
+    if (isError(x)) { 
+        x.message;
+        x.mesage;
+          ~~~~~~
+!!! error TS2339: Property 'mesage' does not exist on type 'Error'.
+    }
+    
+    if (isDate(x)) {
+        x.getDate();
+        x.getHuors();
+          ~~~~~~~~
+!!! error TS2339: Property 'getHuors' does not exist on type 'Date'.
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/narrowFromAnyWithTypePredicate.js b/tests/baselines/reference/narrowFromAnyWithTypePredicate.js
new file mode 100644
index 00000000000..958a3cfd70d
--- /dev/null
+++ b/tests/baselines/reference/narrowFromAnyWithTypePredicate.js
@@ -0,0 +1,60 @@
+//// [narrowFromAnyWithTypePredicate.ts]
+declare var x: any;
+declare function isFunction(x): x is Function;
+declare function isObject(x): x is Object;
+declare function isAnything(x): x is {};
+declare function isError(x): x is Error;
+declare function isDate(x): x is Date;
+
+
+if (isFunction(x)) { // 'any' is not narrowed when target type is 'Function'
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+
+if (isObject(x)) { // 'any' is not narrowed when target type is 'Object'
+    x.method();
+    x();
+}
+
+if (isAnything(x)) { // 'any' is narrowed to types other than 'Function'/'Object' (including {})
+    x.method();
+    x();
+}
+
+if (isError(x)) { 
+    x.message;
+    x.mesage;
+}
+
+if (isDate(x)) {
+    x.getDate();
+    x.getHuors();
+}
+
+
+//// [narrowFromAnyWithTypePredicate.js]
+if (isFunction(x)) {
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+if (isObject(x)) {
+    x.method();
+    x();
+}
+if (isAnything(x)) {
+    x.method();
+    x();
+}
+if (isError(x)) {
+    x.message;
+    x.mesage;
+}
+if (isDate(x)) {
+    x.getDate();
+    x.getHuors();
+}
diff --git a/tests/baselines/reference/stringLiteralTypesAndTuples01.types b/tests/baselines/reference/stringLiteralTypesAndTuples01.types
index 0900d9fc745..10a30a6d75c 100644
--- a/tests/baselines/reference/stringLiteralTypesAndTuples01.types
+++ b/tests/baselines/reference/stringLiteralTypesAndTuples01.types
@@ -55,5 +55,5 @@ function rawr(dino: RexOrRaptor) {
     throw "Unexpected " + dino;
 >"Unexpected " + dino : string
 >"Unexpected " : string
->dino : "t-rex"
+>dino : never
 }
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags01.types b/tests/baselines/reference/stringLiteralTypesAsTags01.types
index e00fd18163d..e95a7364f93 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags01.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags01.types
@@ -99,8 +99,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags02.types b/tests/baselines/reference/stringLiteralTypesAsTags02.types
index fb1632559ea..c984b8a78f9 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags02.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags02.types
@@ -93,8 +93,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/baselines/reference/stringLiteralTypesAsTags03.types b/tests/baselines/reference/stringLiteralTypesAsTags03.types
index 05be633813b..fbe71ff07c1 100644
--- a/tests/baselines/reference/stringLiteralTypesAsTags03.types
+++ b/tests/baselines/reference/stringLiteralTypesAsTags03.types
@@ -96,8 +96,8 @@ if (hasKind(x, "A")) {
 }
 else {
     let b = x;
->b : A
->x : A
+>b : never
+>x : never
 }
 
 if (!hasKind(x, "B")) {
diff --git a/tests/baselines/reference/thisInTupleTypeParameterConstraints.js b/tests/baselines/reference/thisInTupleTypeParameterConstraints.js
new file mode 100644
index 00000000000..c63fbf61991
--- /dev/null
+++ b/tests/baselines/reference/thisInTupleTypeParameterConstraints.js
@@ -0,0 +1,29 @@
+//// [thisInTupleTypeParameterConstraints.ts]
+/// <reference no-default-lib="true"/>
+
+interface Boolean {}
+interface IArguments {}
+interface Function {}
+interface Number {}
+interface RegExp {}
+interface Object {}
+interface String {}
+
+interface Array<T> {
+    // 4 methods will run out of memory if this-types are not instantiated
+    // correctly for tuple types that are type parameter constraints
+    map<U>(arg: this): void;
+    reduceRight<U>(arg: this): void;
+    reduce<U>(arg: this): void;
+    reduce2<U>(arg: this): void;
+}
+
+declare function f<T extends [(x: number) => number]>(a: T): void;
+let x: [(x: number) => number];
+f(x);
+
+
+//// [thisInTupleTypeParameterConstraints.js]
+/// <reference no-default-lib="true"/>
+var x;
+f(x);
diff --git a/tests/baselines/reference/thisInTupleTypeParameterConstraints.symbols b/tests/baselines/reference/thisInTupleTypeParameterConstraints.symbols
new file mode 100644
index 00000000000..4d2bef22c55
--- /dev/null
+++ b/tests/baselines/reference/thisInTupleTypeParameterConstraints.symbols
@@ -0,0 +1,66 @@
+=== tests/cases/compiler/thisInTupleTypeParameterConstraints.ts ===
+/// <reference no-default-lib="true"/>
+
+interface Boolean {}
+>Boolean : Symbol(Boolean, Decl(thisInTupleTypeParameterConstraints.ts, 0, 0))
+
+interface IArguments {}
+>IArguments : Symbol(IArguments, Decl(thisInTupleTypeParameterConstraints.ts, 2, 20))
+
+interface Function {}
+>Function : Symbol(Function, Decl(thisInTupleTypeParameterConstraints.ts, 3, 23))
+
+interface Number {}
+>Number : Symbol(Number, Decl(thisInTupleTypeParameterConstraints.ts, 4, 21))
+
+interface RegExp {}
+>RegExp : Symbol(RegExp, Decl(thisInTupleTypeParameterConstraints.ts, 5, 19))
+
+interface Object {}
+>Object : Symbol(Object, Decl(thisInTupleTypeParameterConstraints.ts, 6, 19))
+
+interface String {}
+>String : Symbol(String, Decl(thisInTupleTypeParameterConstraints.ts, 7, 19))
+
+interface Array<T> {
+>Array : Symbol(Array, Decl(thisInTupleTypeParameterConstraints.ts, 8, 19))
+>T : Symbol(T, Decl(thisInTupleTypeParameterConstraints.ts, 10, 16))
+
+    // 4 methods will run out of memory if this-types are not instantiated
+    // correctly for tuple types that are type parameter constraints
+    map<U>(arg: this): void;
+>map : Symbol(Array.map, Decl(thisInTupleTypeParameterConstraints.ts, 10, 20))
+>U : Symbol(U, Decl(thisInTupleTypeParameterConstraints.ts, 13, 8))
+>arg : Symbol(arg, Decl(thisInTupleTypeParameterConstraints.ts, 13, 11))
+
+    reduceRight<U>(arg: this): void;
+>reduceRight : Symbol(Array.reduceRight, Decl(thisInTupleTypeParameterConstraints.ts, 13, 28))
+>U : Symbol(U, Decl(thisInTupleTypeParameterConstraints.ts, 14, 16))
+>arg : Symbol(arg, Decl(thisInTupleTypeParameterConstraints.ts, 14, 19))
+
+    reduce<U>(arg: this): void;
+>reduce : Symbol(Array.reduce, Decl(thisInTupleTypeParameterConstraints.ts, 14, 36))
+>U : Symbol(U, Decl(thisInTupleTypeParameterConstraints.ts, 15, 11))
+>arg : Symbol(arg, Decl(thisInTupleTypeParameterConstraints.ts, 15, 14))
+
+    reduce2<U>(arg: this): void;
+>reduce2 : Symbol(Array.reduce2, Decl(thisInTupleTypeParameterConstraints.ts, 15, 31))
+>U : Symbol(U, Decl(thisInTupleTypeParameterConstraints.ts, 16, 12))
+>arg : Symbol(arg, Decl(thisInTupleTypeParameterConstraints.ts, 16, 15))
+}
+
+declare function f<T extends [(x: number) => number]>(a: T): void;
+>f : Symbol(f, Decl(thisInTupleTypeParameterConstraints.ts, 17, 1))
+>T : Symbol(T, Decl(thisInTupleTypeParameterConstraints.ts, 19, 19))
+>x : Symbol(x, Decl(thisInTupleTypeParameterConstraints.ts, 19, 31))
+>a : Symbol(a, Decl(thisInTupleTypeParameterConstraints.ts, 19, 54))
+>T : Symbol(T, Decl(thisInTupleTypeParameterConstraints.ts, 19, 19))
+
+let x: [(x: number) => number];
+>x : Symbol(x, Decl(thisInTupleTypeParameterConstraints.ts, 20, 3))
+>x : Symbol(x, Decl(thisInTupleTypeParameterConstraints.ts, 20, 9))
+
+f(x);
+>f : Symbol(f, Decl(thisInTupleTypeParameterConstraints.ts, 17, 1))
+>x : Symbol(x, Decl(thisInTupleTypeParameterConstraints.ts, 20, 3))
+
diff --git a/tests/baselines/reference/thisInTupleTypeParameterConstraints.types b/tests/baselines/reference/thisInTupleTypeParameterConstraints.types
new file mode 100644
index 00000000000..7daafe02bfc
--- /dev/null
+++ b/tests/baselines/reference/thisInTupleTypeParameterConstraints.types
@@ -0,0 +1,67 @@
+=== tests/cases/compiler/thisInTupleTypeParameterConstraints.ts ===
+/// <reference no-default-lib="true"/>
+
+interface Boolean {}
+>Boolean : Boolean
+
+interface IArguments {}
+>IArguments : IArguments
+
+interface Function {}
+>Function : Function
+
+interface Number {}
+>Number : Number
+
+interface RegExp {}
+>RegExp : RegExp
+
+interface Object {}
+>Object : Object
+
+interface String {}
+>String : String
+
+interface Array<T> {
+>Array : T[]
+>T : T
+
+    // 4 methods will run out of memory if this-types are not instantiated
+    // correctly for tuple types that are type parameter constraints
+    map<U>(arg: this): void;
+>map : <U>(arg: this) => void
+>U : U
+>arg : this
+
+    reduceRight<U>(arg: this): void;
+>reduceRight : <U>(arg: this) => void
+>U : U
+>arg : this
+
+    reduce<U>(arg: this): void;
+>reduce : <U>(arg: this) => void
+>U : U
+>arg : this
+
+    reduce2<U>(arg: this): void;
+>reduce2 : <U>(arg: this) => void
+>U : U
+>arg : this
+}
+
+declare function f<T extends [(x: number) => number]>(a: T): void;
+>f : <T extends [(x: number) => number]>(a: T) => void
+>T : T
+>x : number
+>a : T
+>T : T
+
+let x: [(x: number) => number];
+>x : [(x: number) => number]
+>x : number
+
+f(x);
+>f(x) : void
+>f : <T extends [(x: number) => number]>(a: T) => void
+>x : [(x: number) => number]
+
diff --git a/tests/baselines/reference/tsxAttributeResolution14.errors.txt b/tests/baselines/reference/tsxAttributeResolution14.errors.txt
new file mode 100644
index 00000000000..81d42e49059
--- /dev/null
+++ b/tests/baselines/reference/tsxAttributeResolution14.errors.txt
@@ -0,0 +1,38 @@
+tests/cases/conformance/jsx/file.tsx(14,28): error TS2322: Type 'number' is not assignable to type 'string'.
+tests/cases/conformance/jsx/file.tsx(16,28): error TS2322: Type 'boolean' is not assignable to type 'string | number'.
+
+
+==== tests/cases/conformance/jsx/react.d.ts (0 errors) ====
+    
+    declare module JSX {
+    	interface Element { }
+    	interface IntrinsicElements {
+            div: any;
+    	}
+        interface ElementAttributesProperty { prop: any }
+    }
+    
+==== tests/cases/conformance/jsx/file.tsx (2 errors) ====
+    
+    interface IProps {
+      primaryText: string,
+      [propName: string]: string | number
+    }
+    
+    function VerticalNavMenuItem(prop: IProps) {
+      return <div>props.primaryText</div>
+    }
+    
+    function VerticalNav() {
+      return (
+        <div>
+          <VerticalNavMenuItem primaryText={2} />  // error
+                               ~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'number' is not assignable to type 'string'.
+          <VerticalNavMenuItem justRandomProp={2} primaryText={"hello"} />  // ok
+          <VerticalNavMenuItem justRandomProp1={true} primaryText={"hello"} />  // error
+                               ~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2322: Type 'boolean' is not assignable to type 'string | number'.
+        </div>
+      )
+    } 
\ No newline at end of file
diff --git a/tests/baselines/reference/tsxAttributeResolution14.js b/tests/baselines/reference/tsxAttributeResolution14.js
new file mode 100644
index 00000000000..d920179458c
--- /dev/null
+++ b/tests/baselines/reference/tsxAttributeResolution14.js
@@ -0,0 +1,47 @@
+//// [tests/cases/conformance/jsx/tsxAttributeResolution14.tsx] ////
+
+//// [react.d.ts]
+
+declare module JSX {
+	interface Element { }
+	interface IntrinsicElements {
+        div: any;
+	}
+    interface ElementAttributesProperty { prop: any }
+}
+
+//// [file.tsx]
+
+interface IProps {
+  primaryText: string,
+  [propName: string]: string | number
+}
+
+function VerticalNavMenuItem(prop: IProps) {
+  return <div>props.primaryText</div>
+}
+
+function VerticalNav() {
+  return (
+    <div>
+      <VerticalNavMenuItem primaryText={2} />  // error
+      <VerticalNavMenuItem justRandomProp={2} primaryText={"hello"} />  // ok
+      <VerticalNavMenuItem justRandomProp1={true} primaryText={"hello"} />  // error
+    </div>
+  )
+} 
+
+//// [file.jsx]
+function VerticalNavMenuItem(prop) {
+    return <div>props.primaryText</div>;
+}
+function VerticalNav() {
+    return (<div>
+      <VerticalNavMenuItem primaryText={2}/> // error
+      // error
+      <VerticalNavMenuItem justRandomProp={2} primaryText={"hello"}/> // ok
+      // ok
+      <VerticalNavMenuItem justRandomProp1={true} primaryText={"hello"}/> // error
+      // error
+    </div>);
+}
diff --git a/tests/baselines/reference/tsxAttributeResolution5.errors.txt b/tests/baselines/reference/tsxAttributeResolution5.errors.txt
index d7aa46bc05a..42dabc741ab 100644
--- a/tests/baselines/reference/tsxAttributeResolution5.errors.txt
+++ b/tests/baselines/reference/tsxAttributeResolution5.errors.txt
@@ -2,9 +2,10 @@ tests/cases/conformance/jsx/file.tsx(21,16): error TS2606: Property 'x' of JSX s
   Type 'number' is not assignable to type 'string'.
 tests/cases/conformance/jsx/file.tsx(25,9): error TS2324: Property 'x' is missing in type 'Attribs1'.
 tests/cases/conformance/jsx/file.tsx(29,1): error TS2324: Property 'x' is missing in type 'Attribs1'.
+tests/cases/conformance/jsx/file.tsx(30,1): error TS2324: Property 'toString' is missing in type 'Attribs2'.
 
 
-==== tests/cases/conformance/jsx/file.tsx (3 errors) ====
+==== tests/cases/conformance/jsx/file.tsx (4 errors) ====
     declare module JSX {
     	interface Element { }
     	interface IntrinsicElements {
@@ -41,5 +42,7 @@ tests/cases/conformance/jsx/file.tsx(29,1): error TS2324: Property 'x' is missin
     <test1 {...{}} />; // Error, missing x
     ~~~~~~~~~~~~~~~~~
 !!! error TS2324: Property 'x' is missing in type 'Attribs1'.
-    <test2 {...{}} />; // OK
+    <test2 {...{}} />; // Error, missing toString
+    ~~~~~~~~~~~~~~~~~
+!!! error TS2324: Property 'toString' is missing in type 'Attribs2'.
     
\ No newline at end of file
diff --git a/tests/baselines/reference/tsxAttributeResolution5.js b/tests/baselines/reference/tsxAttributeResolution5.js
index abfb54e000c..4bde09074b5 100644
--- a/tests/baselines/reference/tsxAttributeResolution5.js
+++ b/tests/baselines/reference/tsxAttributeResolution5.js
@@ -28,7 +28,7 @@ function make3<T extends {y: string}> (obj: T) {
 
 
 <test1 {...{}} />; // Error, missing x
-<test2 {...{}} />; // OK
+<test2 {...{}} />; // Error, missing toString
 
 
 //// [file.jsx]
@@ -42,4 +42,4 @@ function make3(obj) {
     return <test1 {...obj}/>; // Error, missing x
 }
 <test1 {...{}}/>; // Error, missing x
-<test2 {...{}}/>; // OK
+<test2 {...{}}/>; // Error, missing toString
diff --git a/tests/baselines/reference/typeGuardTautologicalConsistiency.types b/tests/baselines/reference/typeGuardTautologicalConsistiency.types
index f86a9e66fa2..0a6524dd26f 100644
--- a/tests/baselines/reference/typeGuardTautologicalConsistiency.types
+++ b/tests/baselines/reference/typeGuardTautologicalConsistiency.types
@@ -15,7 +15,7 @@ if (typeof stringOrNumber === "number") {
 >"number" : "number"
 
         stringOrNumber;
->stringOrNumber : string
+>stringOrNumber : never
     }
 }
 
@@ -31,6 +31,6 @@ if (typeof stringOrNumber === "number" && typeof stringOrNumber !== "number") {
 >"number" : "number"
 
     stringOrNumber;
->stringOrNumber : string
+>stringOrNumber : never
 }
 
diff --git a/tests/baselines/reference/typeGuardTypeOfUndefined.types b/tests/baselines/reference/typeGuardTypeOfUndefined.types
index 8e9928896a1..520d045d9e0 100644
--- a/tests/baselines/reference/typeGuardTypeOfUndefined.types
+++ b/tests/baselines/reference/typeGuardTypeOfUndefined.types
@@ -47,7 +47,7 @@ function test2(a: any) {
 >"boolean" : "boolean"
 
             a;
->a : boolean
+>a : never
         }
         else {
             a;
@@ -129,7 +129,7 @@ function test5(a: boolean | void) {
         }
         else {
             a;
->a : void
+>a : never
         }
     }
     else {
@@ -188,7 +188,7 @@ function test7(a: boolean | void) {
     }
     else {
         a;
->a : void
+>a : never
     }
 }
 
diff --git a/tests/baselines/reference/typeGuardsAsAssertions.types b/tests/baselines/reference/typeGuardsAsAssertions.types
index 34d3b296a48..661ce9fbc55 100644
--- a/tests/baselines/reference/typeGuardsAsAssertions.types
+++ b/tests/baselines/reference/typeGuardsAsAssertions.types
@@ -193,10 +193,10 @@ function f1() {
 >x : undefined
 
         x;  // string | number (guard as assertion)
->x : string | number
+>x : never
     }
     x;  // string | number | undefined
->x : string | number | undefined
+>x : undefined
 }
 
 function f2() {
@@ -216,10 +216,10 @@ function f2() {
 >"string" : "string"
 
         x;  // string (guard as assertion)
->x : string
+>x : never
     }
     x;  // string | undefined
->x : string | undefined
+>x : undefined
 }
 
 function f3() {
@@ -239,7 +239,7 @@ function f3() {
         return;
     }
     x;  // string | number (guard as assertion)
->x : string | number
+>x : never
 }
 
 function f4() {
@@ -281,7 +281,7 @@ function f5(x: string | number) {
 >"number" : "number"
 
         x;  // number (guard as assertion)
->x : number
+>x : never
     }
     else {
         x;  // string | number
diff --git a/tests/baselines/reference/typeGuardsInIfStatement.errors.txt b/tests/baselines/reference/typeGuardsInIfStatement.errors.txt
index 984ca454e76..cd9ae40932a 100644
--- a/tests/baselines/reference/typeGuardsInIfStatement.errors.txt
+++ b/tests/baselines/reference/typeGuardsInIfStatement.errors.txt
@@ -1,9 +1,10 @@
 tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts(22,10): error TS2354: No best common type exists among return expressions.
 tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts(31,10): error TS2354: No best common type exists among return expressions.
 tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts(49,10): error TS2354: No best common type exists among return expressions.
+tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts(139,17): error TS2339: Property 'toString' does not exist on type 'never'.
 
 
-==== tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts (3 errors) ====
+==== tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts (4 errors) ====
     // In the true branch statement of an 'if' statement, 
     // the type of a variable or parameter is narrowed by any type guard in the 'if' condition when true.
     // In the false branch statement of an 'if' statement, 
@@ -149,5 +150,7 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsInIfStatement.ts(49,10)
             return typeof x === "number"
                 ? x.toString() // number
                 : x.toString(); // boolean | string
+                    ~~~~~~~~
+!!! error TS2339: Property 'toString' does not exist on type 'never'.
         }
     }
\ No newline at end of file
diff --git a/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.errors.txt b/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.errors.txt
index c574118f91d..b67f7328856 100644
--- a/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.errors.txt
+++ b/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.errors.txt
@@ -1,16 +1,26 @@
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(12,10): error TS2339: Property 'bar' does not exist on type 'A'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(18,10): error TS2339: Property 'bar' does not exist on type 'A'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(33,5): error TS2322: Type 'string' is not assignable to type 'number'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(34,10): error TS2339: Property 'bar' does not exist on type 'B<number>'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(41,10): error TS2339: Property 'bar' does not exist on type 'B<any>'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(66,10): error TS2339: Property 'bar2' does not exist on type 'C1'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(72,10): error TS2339: Property 'bar1' does not exist on type 'C1 | C2'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(73,10): error TS2339: Property 'bar2' does not exist on type 'C1 | C2'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(85,10): error TS2339: Property 'bar' does not exist on type 'D'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(91,10): error TS2339: Property 'bar' does not exist on type 'D'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(112,10): error TS2339: Property 'bar2' does not exist on type 'E1'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(118,11): error TS2339: Property 'bar1' does not exist on type 'E1 | E2'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(119,11): error TS2339: Property 'bar2' does not exist on type 'E1 | E2'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(134,11): error TS2339: Property 'foo' does not exist on type 'string | F'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(135,11): error TS2339: Property 'bar' does not exist on type 'string | F'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(160,11): error TS2339: Property 'foo2' does not exist on type 'G1'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(166,11): error TS2339: Property 'foo2' does not exist on type 'G1'.
 tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(182,11): error TS2339: Property 'bar' does not exist on type 'H'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(187,11): error TS2339: Property 'foo1' does not exist on type 'H'.
+tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts(188,11): error TS2339: Property 'foo2' does not exist on type 'H'.
 
 
-==== tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts (10 errors) ====
+==== tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts (20 errors) ====
     interface AConstructor {
         new (): A;
     }
@@ -28,9 +38,11 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj2: any;
-    if (obj2 instanceof A) { // can't narrow type from 'any'
+    if (obj2 instanceof A) {
         obj2.foo;
         obj2.bar;
+             ~~~
+!!! error TS2339: Property 'bar' does not exist on type 'A'.
     }
     
     // a construct signature with generics
@@ -54,10 +66,12 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj4: any;
-    if (obj4 instanceof B) { // can't narrow type from 'any'
+    if (obj4 instanceof B) {
         obj4.foo = "str";
         obj4.foo = 1;
         obj4.bar = "str";
+             ~~~
+!!! error TS2339: Property 'bar' does not exist on type 'B<any>'.
     }
     
     // has multiple construct signature
@@ -88,10 +102,14 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj6: any;
-    if (obj6 instanceof C) { // can't narrow type from 'any'
+    if (obj6 instanceof C) {
         obj6.foo;
         obj6.bar1;
+             ~~~~
+!!! error TS2339: Property 'bar1' does not exist on type 'C1 | C2'.
         obj6.bar2;
+             ~~~~
+!!! error TS2339: Property 'bar2' does not exist on type 'C1 | C2'.
     }
     
     // with object type literal
@@ -109,9 +127,11 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj8: any;
-    if (obj8 instanceof D) { // can't narrow type from 'any'
+    if (obj8 instanceof D) {
         obj8.foo;
         obj8.bar;
+             ~~~
+!!! error TS2339: Property 'bar' does not exist on type 'D'.
     }
     
     // a construct signature that returns a union type
@@ -138,10 +158,14 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj10: any;
-    if (obj10 instanceof E) { // can't narrow type from 'any'
+    if (obj10 instanceof E) {
         obj10.foo;
         obj10.bar1;
+              ~~~~
+!!! error TS2339: Property 'bar1' does not exist on type 'E1 | E2'.
         obj10.bar2;
+              ~~~~
+!!! error TS2339: Property 'bar2' does not exist on type 'E1 | E2'.
     }
     
     // a construct signature that returns any
@@ -165,7 +189,7 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj12: any;
-    if (obj12 instanceof F) { // can't narrow type from 'any'
+    if (obj12 instanceof F) {
         obj12.foo;
         obj12.bar;
     }
@@ -192,9 +216,11 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj14: any;
-    if (obj14 instanceof G) { // can't narrow type from 'any'
+    if (obj14 instanceof G) {
         obj14.foo1;
         obj14.foo2;
+              ~~~~
+!!! error TS2339: Property 'foo2' does not exist on type 'G1'.
     }
     
     // a type with a prototype that has any type
@@ -216,8 +242,24 @@ tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstru
     }
     
     var obj16: any;
-    if (obj16 instanceof H) { // can't narrow type from 'any'
+    if (obj16 instanceof H) {
         obj16.foo1;
+              ~~~~
+!!! error TS2339: Property 'foo1' does not exist on type 'H'.
         obj16.foo2;
+              ~~~~
+!!! error TS2339: Property 'foo2' does not exist on type 'H'.
+    }
+    
+    var obj17: any;
+    if (obj17 instanceof Object) { // can't narrow type from 'any' to 'Object'
+        obj17.foo1;
+        obj17.foo2;
+    }
+    
+    var obj18: any;
+    if (obj18 instanceof Function) { // can't narrow type from 'any' to 'Function'
+        obj18.foo1;
+        obj18.foo2;
     }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.js b/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.js
index 7e6b3324470..40ef6587e75 100644
--- a/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.js
+++ b/tests/baselines/reference/typeGuardsWithInstanceOfByConstructorSignature.js
@@ -14,7 +14,7 @@ if (obj1 instanceof A) { // narrowed to A.
 }
 
 var obj2: any;
-if (obj2 instanceof A) { // can't narrow type from 'any'
+if (obj2 instanceof A) {
     obj2.foo;
     obj2.bar;
 }
@@ -36,7 +36,7 @@ if (obj3 instanceof B) { // narrowed to B<number>.
 }
 
 var obj4: any;
-if (obj4 instanceof B) { // can't narrow type from 'any'
+if (obj4 instanceof B) {
     obj4.foo = "str";
     obj4.foo = 1;
     obj4.bar = "str";
@@ -68,7 +68,7 @@ if (obj5 instanceof C) { // narrowed to C1|C2.
 }
 
 var obj6: any;
-if (obj6 instanceof C) { // can't narrow type from 'any'
+if (obj6 instanceof C) {
     obj6.foo;
     obj6.bar1;
     obj6.bar2;
@@ -87,7 +87,7 @@ if (obj7 instanceof D) { // narrowed to D.
 }
 
 var obj8: any;
-if (obj8 instanceof D) { // can't narrow type from 'any'
+if (obj8 instanceof D) {
     obj8.foo;
     obj8.bar;
 }
@@ -114,7 +114,7 @@ if (obj9 instanceof E) { // narrowed to E1 | E2
 }
 
 var obj10: any;
-if (obj10 instanceof E) { // can't narrow type from 'any'
+if (obj10 instanceof E) {
     obj10.foo;
     obj10.bar1;
     obj10.bar2;
@@ -137,7 +137,7 @@ if (obj11 instanceof F) { // can't type narrowing, construct signature returns a
 }
 
 var obj12: any;
-if (obj12 instanceof F) { // can't narrow type from 'any'
+if (obj12 instanceof F) {
     obj12.foo;
     obj12.bar;
 }
@@ -162,7 +162,7 @@ if (obj13 instanceof G) { // narrowed to G1. G1 is return type of prototype prop
 }
 
 var obj14: any;
-if (obj14 instanceof G) { // can't narrow type from 'any'
+if (obj14 instanceof G) {
     obj14.foo1;
     obj14.foo2;
 }
@@ -184,10 +184,22 @@ if (obj15 instanceof H) { // narrowed to H.
 }
 
 var obj16: any;
-if (obj16 instanceof H) { // can't narrow type from 'any'
+if (obj16 instanceof H) {
     obj16.foo1;
     obj16.foo2;
 }
+
+var obj17: any;
+if (obj17 instanceof Object) { // can't narrow type from 'any' to 'Object'
+    obj17.foo1;
+    obj17.foo2;
+}
+
+var obj18: any;
+if (obj18 instanceof Function) { // can't narrow type from 'any' to 'Function'
+    obj18.foo1;
+    obj18.foo2;
+}
 
 
 //// [typeGuardsWithInstanceOfByConstructorSignature.js]
@@ -278,3 +290,13 @@ if (obj16 instanceof H) {
     obj16.foo1;
     obj16.foo2;
 }
+var obj17;
+if (obj17 instanceof Object) {
+    obj17.foo1;
+    obj17.foo2;
+}
+var obj18;
+if (obj18 instanceof Function) {
+    obj18.foo1;
+    obj18.foo2;
+}
diff --git a/tests/baselines/reference/typeReferenceDirectives1.js b/tests/baselines/reference/typeReferenceDirectives1.js
index 775af9c5283..a6210819931 100644
--- a/tests/baselines/reference/typeReferenceDirectives1.js
+++ b/tests/baselines/reference/typeReferenceDirectives1.js
@@ -2,7 +2,6 @@
 
 //// [index.d.ts]
 
-
 interface $ { x }
 
 //// [app.ts]
diff --git a/tests/baselines/reference/typeReferenceDirectives1.symbols b/tests/baselines/reference/typeReferenceDirectives1.symbols
index 55c17b219ec..a33a2aba408 100644
--- a/tests/baselines/reference/typeReferenceDirectives1.symbols
+++ b/tests/baselines/reference/typeReferenceDirectives1.symbols
@@ -9,8 +9,7 @@ interface A {
 }
 === /types/lib/index.d.ts ===
 
-
 interface $ { x }
 >$ : Symbol($, Decl(index.d.ts, 0, 0))
->x : Symbol($.x, Decl(index.d.ts, 2, 13))
+>x : Symbol($.x, Decl(index.d.ts, 1, 13))
 
diff --git a/tests/baselines/reference/typeReferenceDirectives1.types b/tests/baselines/reference/typeReferenceDirectives1.types
index 05080e05651..be789a08ddc 100644
--- a/tests/baselines/reference/typeReferenceDirectives1.types
+++ b/tests/baselines/reference/typeReferenceDirectives1.types
@@ -9,7 +9,6 @@ interface A {
 }
 === /types/lib/index.d.ts ===
 
-
 interface $ { x }
 >$ : $
 >x : any
diff --git a/tests/baselines/reference/typingsLookup2.js b/tests/baselines/reference/typingsLookup2.js
new file mode 100644
index 00000000000..3e816526af2
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup2.js
@@ -0,0 +1,9 @@
+//// [tests/cases/conformance/typings/typingsLookup2.ts] ////
+
+//// [package.json]
+{ "typings": null }
+
+//// [a.ts]
+
+
+//// [a.js]
diff --git a/tests/baselines/reference/typingsLookup2.symbols b/tests/baselines/reference/typingsLookup2.symbols
new file mode 100644
index 00000000000..7223c8589a6
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup2.symbols
@@ -0,0 +1,3 @@
+=== /a.ts ===
+
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/typingsLookup2.trace.json b/tests/baselines/reference/typingsLookup2.trace.json
new file mode 100644
index 00000000000..0637a088a01
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup2.trace.json
@@ -0,0 +1 @@
+[]
\ No newline at end of file
diff --git a/tests/baselines/reference/typingsLookup2.types b/tests/baselines/reference/typingsLookup2.types
new file mode 100644
index 00000000000..7223c8589a6
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup2.types
@@ -0,0 +1,3 @@
+=== /a.ts ===
+
+No type information for this code.
\ No newline at end of file
diff --git a/tests/baselines/reference/typingsLookup3.js b/tests/baselines/reference/typingsLookup3.js
new file mode 100644
index 00000000000..b3be036b4ae
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup3.js
@@ -0,0 +1,13 @@
+//// [tests/cases/conformance/typings/typingsLookup3.ts] ////
+
+//// [index.d.ts]
+declare var $: { x: any };
+
+//// [a.ts]
+/// <reference types="JqUeRy" />
+$.x;
+
+
+//// [a.js]
+/// <reference types="JqUeRy" />
+$.x;
diff --git a/tests/baselines/reference/typingsLookup3.symbols b/tests/baselines/reference/typingsLookup3.symbols
new file mode 100644
index 00000000000..e641afb183b
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup3.symbols
@@ -0,0 +1,12 @@
+=== /a.ts ===
+/// <reference types="JqUeRy" />
+$.x;
+>$.x : Symbol(x, Decl(index.d.ts, 0, 16))
+>$ : Symbol($, Decl(index.d.ts, 0, 11))
+>x : Symbol(x, Decl(index.d.ts, 0, 16))
+
+=== /node_modules/@types/jquery/index.d.ts ===
+declare var $: { x: any };
+>$ : Symbol($, Decl(index.d.ts, 0, 11))
+>x : Symbol(x, Decl(index.d.ts, 0, 16))
+
diff --git a/tests/baselines/reference/typingsLookup3.trace.json b/tests/baselines/reference/typingsLookup3.trace.json
new file mode 100644
index 00000000000..83b0e91d6c7
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup3.trace.json
@@ -0,0 +1,12 @@
+[
+    "======== Resolving type reference directive 'jquery', containing file '/a.ts', root directory '/node_modules/@types'. ========",
+    "Resolving with primary search path '/node_modules/@types'",
+    "File '/node_modules/@types/jquery/package.json' does not exist.",
+    "File '/node_modules/@types/jquery/index.d.ts' exist - use it as a name resolution result.",
+    "======== Type reference directive 'jquery' was successfully resolved to '/node_modules/@types/jquery/index.d.ts', primary: true. ========",
+    "======== Resolving type reference directive 'jquery', containing file '/__inferred type names__.ts', root directory '/node_modules/@types'. ========",
+    "Resolving with primary search path '/node_modules/@types'",
+    "File '/node_modules/@types/jquery/package.json' does not exist.",
+    "File '/node_modules/@types/jquery/index.d.ts' exist - use it as a name resolution result.",
+    "======== Type reference directive 'jquery' was successfully resolved to '/node_modules/@types/jquery/index.d.ts', primary: true. ========"
+]
\ No newline at end of file
diff --git a/tests/baselines/reference/typingsLookup3.types b/tests/baselines/reference/typingsLookup3.types
new file mode 100644
index 00000000000..f57a974077e
--- /dev/null
+++ b/tests/baselines/reference/typingsLookup3.types
@@ -0,0 +1,12 @@
+=== /a.ts ===
+/// <reference types="JqUeRy" />
+$.x;
+>$.x : any
+>$ : { x: any; }
+>x : any
+
+=== /node_modules/@types/jquery/index.d.ts ===
+declare var $: { x: any };
+>$ : { x: any; }
+>x : any
+
diff --git a/tests/baselines/reference/umd-augmentation-1.symbols b/tests/baselines/reference/umd-augmentation-1.symbols
index 645511350a9..70715649894 100644
--- a/tests/baselines/reference/umd-augmentation-1.symbols
+++ b/tests/baselines/reference/umd-augmentation-1.symbols
@@ -39,6 +39,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
+>Math2d : Symbol(Math2d, Decl(index.d.ts, 0, 0))
 
 export interface Point {
 >Point : Symbol(Point, Decl(index.d.ts, 1, 27))
diff --git a/tests/baselines/reference/umd-augmentation-1.types b/tests/baselines/reference/umd-augmentation-1.types
index 31ac43fe855..59b577141e0 100644
--- a/tests/baselines/reference/umd-augmentation-1.types
+++ b/tests/baselines/reference/umd-augmentation-1.types
@@ -48,7 +48,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
->Math2d : any
+>Math2d : typeof Math2d
 
 export interface Point {
 >Point : Point
diff --git a/tests/baselines/reference/umd-augmentation-2.symbols b/tests/baselines/reference/umd-augmentation-2.symbols
index bd6584d3d74..a31ed61d531 100644
--- a/tests/baselines/reference/umd-augmentation-2.symbols
+++ b/tests/baselines/reference/umd-augmentation-2.symbols
@@ -37,6 +37,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
+>Math2d : Symbol(Math2d, Decl(index.d.ts, 0, 0))
 
 export interface Point {
 >Point : Symbol(Point, Decl(index.d.ts, 1, 27))
diff --git a/tests/baselines/reference/umd-augmentation-2.types b/tests/baselines/reference/umd-augmentation-2.types
index 20bba091903..24fe6df3c3c 100644
--- a/tests/baselines/reference/umd-augmentation-2.types
+++ b/tests/baselines/reference/umd-augmentation-2.types
@@ -46,7 +46,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
->Math2d : any
+>Math2d : typeof Math2d
 
 export interface Point {
 >Point : Point
diff --git a/tests/baselines/reference/umd-augmentation-3.symbols b/tests/baselines/reference/umd-augmentation-3.symbols
index 3193e83cd19..3e2df7c8a89 100644
--- a/tests/baselines/reference/umd-augmentation-3.symbols
+++ b/tests/baselines/reference/umd-augmentation-3.symbols
@@ -39,6 +39,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
+>Math2d : Symbol(Math2d, Decl(index.d.ts, 0, 0))
 
 export = M2D;
 >M2D : Symbol(M2D, Decl(index.d.ts, 3, 13))
diff --git a/tests/baselines/reference/umd-augmentation-3.types b/tests/baselines/reference/umd-augmentation-3.types
index 854ebceff37..39bc28cbd24 100644
--- a/tests/baselines/reference/umd-augmentation-3.types
+++ b/tests/baselines/reference/umd-augmentation-3.types
@@ -48,7 +48,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
->Math2d : any
+>Math2d : typeof Math2d
 
 export = M2D;
 >M2D : typeof M2D
diff --git a/tests/baselines/reference/umd-augmentation-4.symbols b/tests/baselines/reference/umd-augmentation-4.symbols
index 3f2cc913d86..7be5d278bf2 100644
--- a/tests/baselines/reference/umd-augmentation-4.symbols
+++ b/tests/baselines/reference/umd-augmentation-4.symbols
@@ -37,6 +37,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
+>Math2d : Symbol(Math2d, Decl(index.d.ts, 0, 0))
 
 export = M2D;
 >M2D : Symbol(M2D, Decl(index.d.ts, 3, 13))
diff --git a/tests/baselines/reference/umd-augmentation-4.types b/tests/baselines/reference/umd-augmentation-4.types
index 71783d03012..246270aa5a6 100644
--- a/tests/baselines/reference/umd-augmentation-4.types
+++ b/tests/baselines/reference/umd-augmentation-4.types
@@ -46,7 +46,7 @@ var t = p.x;
 === tests/cases/conformance/externalModules/node_modules/math2d/index.d.ts ===
 
 export as namespace Math2d;
->Math2d : any
+>Math2d : typeof Math2d
 
 export = M2D;
 >M2D : typeof M2D
diff --git a/tests/baselines/reference/umd1.symbols b/tests/baselines/reference/umd1.symbols
index 9b964456bcf..0307a9b78d3 100644
--- a/tests/baselines/reference/umd1.symbols
+++ b/tests/baselines/reference/umd1.symbols
@@ -30,4 +30,5 @@ export interface Thing { n: typeof x }
 >x : Symbol(x, Decl(foo.d.ts, 1, 10))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 3, 38))
 
diff --git a/tests/baselines/reference/umd1.types b/tests/baselines/reference/umd1.types
index 1767f3b5a89..1379bba8d5d 100644
--- a/tests/baselines/reference/umd1.types
+++ b/tests/baselines/reference/umd1.types
@@ -31,5 +31,5 @@ export interface Thing { n: typeof x }
 >x : number
 
 export as namespace Foo;
->Foo : any
+>Foo : typeof Foo
 
diff --git a/tests/baselines/reference/umd3.symbols b/tests/baselines/reference/umd3.symbols
index 165fd81597a..c1fd3d6d74f 100644
--- a/tests/baselines/reference/umd3.symbols
+++ b/tests/baselines/reference/umd3.symbols
@@ -32,4 +32,5 @@ export interface Thing { n: typeof x }
 >x : Symbol(x, Decl(foo.d.ts, 1, 10))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 3, 38))
 
diff --git a/tests/baselines/reference/umd3.types b/tests/baselines/reference/umd3.types
index 85ee6bafe5e..23149e58d76 100644
--- a/tests/baselines/reference/umd3.types
+++ b/tests/baselines/reference/umd3.types
@@ -33,5 +33,5 @@ export interface Thing { n: typeof x }
 >x : number
 
 export as namespace Foo;
->Foo : any
+>Foo : typeof Foo
 
diff --git a/tests/baselines/reference/umd4.symbols b/tests/baselines/reference/umd4.symbols
index 8403187198b..d266997770b 100644
--- a/tests/baselines/reference/umd4.symbols
+++ b/tests/baselines/reference/umd4.symbols
@@ -32,4 +32,5 @@ export interface Thing { n: typeof x }
 >x : Symbol(x, Decl(foo.d.ts, 1, 10))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 3, 38))
 
diff --git a/tests/baselines/reference/umd4.types b/tests/baselines/reference/umd4.types
index 579599f5661..18794258517 100644
--- a/tests/baselines/reference/umd4.types
+++ b/tests/baselines/reference/umd4.types
@@ -33,5 +33,5 @@ export interface Thing { n: typeof x }
 >x : number
 
 export as namespace Foo;
->Foo : any
+>Foo : typeof Foo
 
diff --git a/tests/baselines/reference/umd6.symbols b/tests/baselines/reference/umd6.symbols
index d08507f1ea2..958875b79eb 100644
--- a/tests/baselines/reference/umd6.symbols
+++ b/tests/baselines/reference/umd6.symbols
@@ -18,4 +18,5 @@ export = Thing;
 >Thing : Symbol(Thing, Decl(foo.d.ts, 0, 0))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 4, 15))
 
diff --git a/tests/baselines/reference/umd6.types b/tests/baselines/reference/umd6.types
index 7318a43057a..5b1469cbd6c 100644
--- a/tests/baselines/reference/umd6.types
+++ b/tests/baselines/reference/umd6.types
@@ -19,5 +19,5 @@ export = Thing;
 >Thing : typeof Thing
 
 export as namespace Foo;
->Foo : any
+>Foo : typeof Thing
 
diff --git a/tests/baselines/reference/umd7.symbols b/tests/baselines/reference/umd7.symbols
index 0b3ef17fb7b..19b56ed30f3 100644
--- a/tests/baselines/reference/umd7.symbols
+++ b/tests/baselines/reference/umd7.symbols
@@ -13,4 +13,5 @@ export = Thing;
 >Thing : Symbol(Thing, Decl(foo.d.ts, 0, 0))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 2, 15))
 
diff --git a/tests/baselines/reference/umd7.types b/tests/baselines/reference/umd7.types
index 60782543710..d83e039b674 100644
--- a/tests/baselines/reference/umd7.types
+++ b/tests/baselines/reference/umd7.types
@@ -14,5 +14,5 @@ export = Thing;
 >Thing : () => number
 
 export as namespace Foo;
->Foo : any
+>Foo : () => number
 
diff --git a/tests/baselines/reference/umd8.symbols b/tests/baselines/reference/umd8.symbols
index 8c38f267a2a..97da693b38f 100644
--- a/tests/baselines/reference/umd8.symbols
+++ b/tests/baselines/reference/umd8.symbols
@@ -22,4 +22,5 @@ export = Thing;
 >Thing : Symbol(Thing, Decl(foo.d.ts, 0, 0))
 
 export as namespace Foo;
+>Foo : Symbol(Foo, Decl(foo.d.ts, 4, 15))
 
diff --git a/tests/baselines/reference/umd8.types b/tests/baselines/reference/umd8.types
index 0e66a49b963..ae3bdfd5fc8 100644
--- a/tests/baselines/reference/umd8.types
+++ b/tests/baselines/reference/umd8.types
@@ -23,5 +23,5 @@ export = Thing;
 >Thing : Thing
 
 export as namespace Foo;
->Foo : any
+>Foo : typeof Thing
 
diff --git a/tests/baselines/reference/unusedParameterProperty1.errors.txt b/tests/baselines/reference/unusedParameterProperty1.errors.txt
new file mode 100644
index 00000000000..0581199313f
--- /dev/null
+++ b/tests/baselines/reference/unusedParameterProperty1.errors.txt
@@ -0,0 +1,14 @@
+tests/cases/compiler/unusedParameterProperty1.ts(3,25): error TS6138: Property 'used' is declared but never used.
+
+
+==== tests/cases/compiler/unusedParameterProperty1.ts (1 errors) ====
+    
+    class A {
+        constructor(private used: string) {
+                            ~~~~
+!!! error TS6138: Property 'used' is declared but never used.
+            let foge = used;
+            foge += "";
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/unusedParameterProperty1.js b/tests/baselines/reference/unusedParameterProperty1.js
new file mode 100644
index 00000000000..d0b608a6d87
--- /dev/null
+++ b/tests/baselines/reference/unusedParameterProperty1.js
@@ -0,0 +1,19 @@
+//// [unusedParameterProperty1.ts]
+
+class A {
+    constructor(private used: string) {
+        let foge = used;
+        foge += "";
+    }
+}
+
+
+//// [unusedParameterProperty1.js]
+var A = (function () {
+    function A(used) {
+        this.used = used;
+        var foge = used;
+        foge += "";
+    }
+    return A;
+}());
diff --git a/tests/baselines/reference/unusedParameterProperty2.errors.txt b/tests/baselines/reference/unusedParameterProperty2.errors.txt
new file mode 100644
index 00000000000..cb3b3e95556
--- /dev/null
+++ b/tests/baselines/reference/unusedParameterProperty2.errors.txt
@@ -0,0 +1,14 @@
+tests/cases/compiler/unusedParameterProperty2.ts(3,25): error TS6138: Property 'used' is declared but never used.
+
+
+==== tests/cases/compiler/unusedParameterProperty2.ts (1 errors) ====
+    
+    class A {
+        constructor(private used) {
+                            ~~~~
+!!! error TS6138: Property 'used' is declared but never used.
+            let foge = used;
+            foge += "";
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/unusedParameterProperty2.js b/tests/baselines/reference/unusedParameterProperty2.js
new file mode 100644
index 00000000000..2bb04fde088
--- /dev/null
+++ b/tests/baselines/reference/unusedParameterProperty2.js
@@ -0,0 +1,19 @@
+//// [unusedParameterProperty2.ts]
+
+class A {
+    constructor(private used) {
+        let foge = used;
+        foge += "";
+    }
+}
+
+
+//// [unusedParameterProperty2.js]
+var A = (function () {
+    function A(used) {
+        this.used = used;
+        var foge = used;
+        foge += "";
+    }
+    return A;
+}());
diff --git a/tests/cases/compiler/APISample_watcher.ts b/tests/cases/compiler/APISample_watcher.ts
index 34baa04c850..07922bd35c7 100644
--- a/tests/cases/compiler/APISample_watcher.ts
+++ b/tests/cases/compiler/APISample_watcher.ts
@@ -23,7 +23,7 @@ declare var path: any;
 import * as ts from "typescript";
 
 function watch(rootFileNames: string[], options: ts.CompilerOptions) {
-    const files: ts.Map<{ version: number }> = {};
+    const files: ts.MapLike<{ version: number }> = {};
 
     // initialize the list of files
     rootFileNames.forEach(fileName => {
diff --git a/tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts b/tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts
new file mode 100644
index 00000000000..d621cf460bc
--- /dev/null
+++ b/tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts
@@ -0,0 +1,18 @@
+class A { a }
+class B extends A { b }
+class C<T> extends Array<T> { c }
+declare var ara: A[];
+declare var arb: B[];
+declare var cra: C<A>;
+declare var crb: C<B>;
+declare var rra: ReadonlyArray<A>;
+declare var rrb: ReadonlyArray<B>;
+rra = ara;
+rrb = arb; // OK, Array<B> is assignable to ReadonlyArray<A>
+rra = arb;
+rrb = ara; // error: 'A' is not assignable to 'B'
+
+rra = cra;
+rra = crb; // OK, C<B> is assignable to ReadonlyArray<A>
+rrb = crb;
+rrb = cra; // error: 'A' is not assignable to 'B'
diff --git a/tests/cases/compiler/controlFlowInstanceof.ts b/tests/cases/compiler/controlFlowInstanceof.ts
new file mode 100644
index 00000000000..56f3ff97e4c
--- /dev/null
+++ b/tests/cases/compiler/controlFlowInstanceof.ts
@@ -0,0 +1,99 @@
+// @target: es6
+
+// Repros from #10167
+
+function f1(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f2(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Promise) {
+        s;  // Set<number> & Promise<any>
+    }
+    s;  // Set<number>
+    s.add(42);
+}
+
+function f3(s: Set<string> | Set<number>) {
+    s;  // Set<string> | Set<number>
+    if (s instanceof Set) {
+        s;  // Set<string> | Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+function f4(s: Set<string> | Set<number>) {
+    s = new Set<number>();
+    s;  // Set<number>
+    if (s instanceof Set) {
+        s;  // Set<number>
+    }
+    else {
+        s;  // never
+    }
+}
+
+// More tests
+
+class A { a: string }
+class B extends A { b: string }
+class C extends A { c: string }
+
+function foo(x: A | undefined) {
+    x;  // A | undefined
+    if (x instanceof B || x instanceof C) {
+        x;  // B | C
+    }
+    x;  // A | undefined
+    if (x instanceof B && x instanceof C) {
+        x;  // B & C
+    }
+    x;  // A | undefined
+    if (!x) {
+        return;
+    }
+    x;  // A
+    if (x instanceof B) {
+        x;  // B
+        if (x instanceof C) {
+            x;  // B & C
+        }
+        else {
+            x;  // B
+        }
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+    x;  // A
+}
+
+// X is neither assignable to Y nor a subtype of Y
+// Y is assignable to X, but not a subtype of X
+
+interface X {
+    x?: string;
+}
+
+class Y {
+    y: string;
+}
+
+function goo(x: X) {
+    x;
+    if (x instanceof Y) {
+        x.y;
+    }
+    x;
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantPropertyCheck.ts b/tests/cases/compiler/discriminantPropertyCheck.ts
new file mode 100644
index 00000000000..e493f6bf770
--- /dev/null
+++ b/tests/cases/compiler/discriminantPropertyCheck.ts
@@ -0,0 +1,69 @@
+// @strictNullChecks: true
+
+type Item = Item1 | Item2;
+
+interface Base {
+    bar: boolean;
+}
+
+interface Item1 extends Base {
+    kind: "A";
+    foo: string | undefined;
+    baz: boolean;
+    qux: true;
+}
+
+interface Item2 extends Base {
+    kind: "B";
+    foo: string | undefined;
+    baz: boolean;
+    qux: false;
+}
+
+function goo1(x: Item) {
+    if (x.kind === "A" && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function goo2(x: Item) {
+    if (x.foo !== undefined && x.kind === "A") {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
+
+function foo1(x: Item) {
+    if (x.bar && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo2(x: Item) {
+    if (x.foo !== undefined && x.bar) {
+        x.foo.length;
+    }
+}
+
+function foo3(x: Item) {
+    if (x.baz && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo4(x: Item) {
+    if (x.foo !== undefined && x.baz) {
+        x.foo.length;
+    }
+}
+
+function foo5(x: Item) {
+    if (x.qux && x.foo !== undefined) {
+        x.foo.length;
+    }
+}
+
+function foo6(x: Item) {
+    if (x.foo !== undefined && x.qux) {
+        x.foo.length;  // Error, intervening discriminant guard
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantsAndNullOrUndefined.ts b/tests/cases/compiler/discriminantsAndNullOrUndefined.ts
new file mode 100644
index 00000000000..8346fa8adea
--- /dev/null
+++ b/tests/cases/compiler/discriminantsAndNullOrUndefined.ts
@@ -0,0 +1,25 @@
+// @strictNullChecks: true
+
+// Repro from #10228
+
+interface A { kind: 'A'; }
+interface B { kind: 'B'; }
+
+type C = A | B | undefined;
+
+function never(_: never): never {
+    throw new Error();
+}
+
+function useA(_: A): void { }
+function useB(_: B): void { }
+
+declare var c: C;
+
+if (c !== undefined) {
+    switch (c.kind) {
+        case 'A': useA(c); break;
+        case 'B': useB(c); break;
+        default: never(c);
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantsAndPrimitives.ts b/tests/cases/compiler/discriminantsAndPrimitives.ts
new file mode 100644
index 00000000000..6352d741808
--- /dev/null
+++ b/tests/cases/compiler/discriminantsAndPrimitives.ts
@@ -0,0 +1,49 @@
+// @strictNullChecks: true
+
+// Repro from #10257 plus other tests
+
+interface Foo {
+    kind: "foo";
+    name: string;
+}
+
+interface Bar {
+    kind: "bar";
+    length: string;
+}
+
+function f1(x: Foo | Bar | string) {
+    if (typeof x !== 'string') {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f2(x: Foo | Bar | string | undefined) {
+    if (typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f3(x: Foo | Bar | string | null) {
+    if (x && typeof x !== "string") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
+
+function f4(x: Foo | Bar | string | number | null) {
+    if (x && typeof x === "object") {
+        switch(x.kind) {
+            case 'foo':
+                x.name;
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/discriminantsAndTypePredicates.ts b/tests/cases/compiler/discriminantsAndTypePredicates.ts
new file mode 100644
index 00000000000..c21ab7ec8f4
--- /dev/null
+++ b/tests/cases/compiler/discriminantsAndTypePredicates.ts
@@ -0,0 +1,31 @@
+// Repro from #10145
+
+interface A { type: 'A' }
+interface B { type: 'B' }
+
+function isA(x: A | B): x is A { return x.type === 'A'; }
+function isB(x: A | B): x is B { return x.type === 'B'; }
+
+function foo1(x: A | B): any {
+    x;  // A | B
+    if (isA(x)) {
+        return x;  // A
+    }
+    x;  // B
+    if (isB(x)) {
+        return x;  // B
+    }
+    x;  // never
+}
+
+function foo2(x: A | B): any {
+    x;  // A | B
+    if (x.type === 'A') {
+        return x;  // A
+    }
+    x;  // B
+    if (x.type === 'B') {
+        return x;  // B
+    }
+    x;  // never
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts b/tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts
new file mode 100644
index 00000000000..e369cd9fb50
--- /dev/null
+++ b/tests/cases/compiler/emitCapturingThisInTupleDestructuring1.ts
@@ -0,0 +1,4 @@
+declare function wrapper(x: any);
+wrapper((array: [any]) => {
+    [this.test, this.test1, this.test2] = array;  // even though there is a compiler error, we should still emit lexical capture for "this"
+});
\ No newline at end of file
diff --git a/tests/cases/compiler/emitCapturingThisInTupleDestructuring2.ts b/tests/cases/compiler/emitCapturingThisInTupleDestructuring2.ts
new file mode 100644
index 00000000000..2bb931c4f4f
--- /dev/null
+++ b/tests/cases/compiler/emitCapturingThisInTupleDestructuring2.ts
@@ -0,0 +1,10 @@
+var array1: [number, number] = [1, 2];
+
+class B {
+    test: number;
+    test1: any;
+    test2: any;
+    method() {
+        () => [this.test, this.test1, this.test2] = array1; // even though there is a compiler error, we should still emit lexical capture for "this" 
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/exportDefaultProperty.ts b/tests/cases/compiler/exportDefaultProperty.ts
new file mode 100644
index 00000000000..4a4b4139025
--- /dev/null
+++ b/tests/cases/compiler/exportDefaultProperty.ts
@@ -0,0 +1,39 @@
+// This test is just like exportEqualsProperty, but with `export default`.
+
+// @Filename: declarations.d.ts
+declare namespace foo.bar {
+    export type X = number;
+    export const X: number;
+}
+
+declare module "foobar" {
+    export default foo.bar;
+}
+
+declare module "foobarx" {
+    export default foo.bar.X;
+}
+
+// @Filename: a.ts
+namespace A {
+    export class B { constructor(b: number) {} }
+    export namespace B { export const b: number = 0; }
+}
+export default A.B;
+
+// @Filename: b.ts
+export default "foo".length;
+
+// @Filename: index.ts
+/// <reference path="declarations.d.ts" />
+import fooBar from "foobar";
+import X = fooBar.X;
+import X2 from "foobarx";
+const x: X = X;
+const x2: X2 = X2;
+
+import B from "./a";
+const b: B = new B(B.b);
+
+import fooLength from "./b";
+fooLength + 1;
diff --git a/tests/cases/compiler/exportEqualsProperty.ts b/tests/cases/compiler/exportEqualsProperty.ts
new file mode 100644
index 00000000000..0d14815a5bd
--- /dev/null
+++ b/tests/cases/compiler/exportEqualsProperty.ts
@@ -0,0 +1,38 @@
+// This test is just like exportDefaultProperty, but with `export =`.
+
+// @Filename: declarations.d.ts
+declare namespace foo.bar {
+    export type X = number;
+    export const X: number;
+}
+
+declare module "foobar" {
+    export = foo.bar;
+}
+
+declare module "foobarx" {
+    export = foo.bar.X;
+}
+
+// @Filename: a.ts
+namespace A {
+    export class B { constructor(b: number) {} }
+    export namespace B { export const b: number = 0; }
+}
+export = A.B;
+
+// @Filename: b.ts
+export = "foo".length;
+
+// @Filename: index.ts
+/// <reference path="declarations.d.ts" />
+import { X } from "foobar";
+import X2 = require("foobarx");
+const x: X = X;
+const x2: X2 = X2;
+
+import B = require("./a");
+const b: B = new B(B.b);
+
+import fooLength = require("./b");
+fooLength + 1;
diff --git a/tests/cases/compiler/exportToString.ts b/tests/cases/compiler/exportToString.ts
new file mode 100644
index 00000000000..248df036bfd
--- /dev/null
+++ b/tests/cases/compiler/exportToString.ts
@@ -0,0 +1,2 @@
+const toString = 0;
+export { toString };
diff --git a/tests/cases/compiler/implicitConstParameters.ts b/tests/cases/compiler/implicitConstParameters.ts
new file mode 100644
index 00000000000..97996789124
--- /dev/null
+++ b/tests/cases/compiler/implicitConstParameters.ts
@@ -0,0 +1,57 @@
+// @strictNullChecks: true
+
+function doSomething(cb: () => void) {
+    cb();
+}
+
+function fn(x: number | string) {
+  if (typeof x === 'number') {
+      doSomething(() => x.toFixed());
+  }
+}
+
+function f1(x: string | undefined) {
+    if (!x) {
+        return;
+    }
+    doSomething(() => x.length);
+}
+
+function f2(x: string | undefined) {
+    if (x) {
+        doSomething(() => {
+            doSomething(() => x.length);
+        });
+    }
+}
+
+function f3(x: string | undefined) {
+    inner();
+    function inner() {
+        if (x) {
+            doSomething(() => x.length);
+        }
+    }
+}
+
+function f4(x: string | undefined) {
+    x = "abc";  // causes x to be considered non-const
+    if (x) {
+        doSomething(() => x.length);
+    }
+}
+
+function f5(x: string | undefined) {
+    if (x) {
+        doSomething(() => x.length);
+    }
+    x = "abc";  // causes x to be considered non-const
+}
+
+
+function f6(x: string | undefined) {
+    const y = x || "";
+    if (x) {
+        doSomething(() => y.length);
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/compiler/indexWithUndefinedAndNull.ts b/tests/cases/compiler/indexWithUndefinedAndNull.ts
new file mode 100644
index 00000000000..2aeb2ee0b1d
--- /dev/null
+++ b/tests/cases/compiler/indexWithUndefinedAndNull.ts
@@ -0,0 +1,13 @@
+// @strictNullChecks: false
+interface N {
+    [n: number]: string;
+}
+interface S {
+    [s: string]: number;
+}
+let n: N;
+let s: S;
+let str: string = n[undefined];
+str = n[null];
+let num: number = s[undefined];
+num = s[null];
diff --git a/tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts b/tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts
new file mode 100644
index 00000000000..f8fe0a323c6
--- /dev/null
+++ b/tests/cases/compiler/indexWithUndefinedAndNullStrictNullChecks.ts
@@ -0,0 +1,13 @@
+// @strictNullChecks: true
+interface N {
+    [n: number]: string;
+}
+interface S {
+    [s: string]: number;
+}
+let n: N;
+let s: S;
+let str: string = n[undefined];
+str = n[null];
+let num: number = s[undefined];
+num = s[null];
diff --git a/tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts b/tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts
new file mode 100644
index 00000000000..564f7a9c22e
--- /dev/null
+++ b/tests/cases/compiler/instanceofWithStructurallyIdenticalTypes.ts
@@ -0,0 +1,69 @@
+// Repro from #7271
+
+class C1 { item: string }
+class C2 { item: string[] }
+class C3 { item: string }
+
+function foo1(x: C1 | C2 | C3): string {
+    if (x instanceof C1) {
+        return x.item;
+    }
+    else if (x instanceof C2) {
+        return x.item[0];
+    }
+    else if (x instanceof C3) {
+        return x.item;
+    }
+    return "error";
+}
+
+function isC1(c: C1 | C2 | C3): c is C1 { return c instanceof C1 }
+function isC2(c: C1 | C2 | C3): c is C2 { return c instanceof C2 }
+function isC3(c: C1 | C2 | C3): c is C3 { return c instanceof C3 }
+
+function foo2(x: C1 | C2 | C3): string {
+    if (isC1(x)) {
+        return x.item;
+    }
+    else if (isC2(x)) {
+        return x.item[0];
+    }
+    else if (isC3(x)) {
+        return x.item;
+    }
+    return "error";
+}
+
+// More tests
+
+class A { a: string }
+class A1 extends A { }
+class A2 { a: string }
+class B extends A { b: string }
+
+function goo(x: A) {
+    if (x instanceof A) {
+        x;  // A
+    }
+    else {
+        x;  // never
+    }
+    if (x instanceof A1) {
+        x;  // A1
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof A2) {
+        x;  // A2
+    }
+    else {
+        x;  // A
+    }
+    if (x instanceof B) {
+        x;  // B
+    }
+    else {
+        x;  // A
+    }
+}
diff --git a/tests/cases/compiler/thisInTupleTypeParameterConstraints.ts b/tests/cases/compiler/thisInTupleTypeParameterConstraints.ts
new file mode 100644
index 00000000000..b6d0d338d85
--- /dev/null
+++ b/tests/cases/compiler/thisInTupleTypeParameterConstraints.ts
@@ -0,0 +1,22 @@
+/// <reference no-default-lib="true"/>
+
+interface Boolean {}
+interface IArguments {}
+interface Function {}
+interface Number {}
+interface RegExp {}
+interface Object {}
+interface String {}
+
+interface Array<T> {
+    // 4 methods will run out of memory if this-types are not instantiated
+    // correctly for tuple types that are type parameter constraints
+    map<U>(arg: this): void;
+    reduceRight<U>(arg: this): void;
+    reduce<U>(arg: this): void;
+    reduce2<U>(arg: this): void;
+}
+
+declare function f<T extends [(x: number) => number]>(a: T): void;
+let x: [(x: number) => number];
+f(x);
diff --git a/tests/cases/compiler/typeReferenceDirectives1.ts b/tests/cases/compiler/typeReferenceDirectives1.ts
index e17e498b978..13c84c7ae5e 100644
--- a/tests/cases/compiler/typeReferenceDirectives1.ts
+++ b/tests/cases/compiler/typeReferenceDirectives1.ts
@@ -2,7 +2,7 @@
 // @traceResolution: true
 // @declaration: true
 // @typeRoots: /types
-
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 interface $ { x }
diff --git a/tests/cases/compiler/typeReferenceDirectives10.ts b/tests/cases/compiler/typeReferenceDirectives10.ts
index 61971ba44b2..1eb796d03fd 100644
--- a/tests/cases/compiler/typeReferenceDirectives10.ts
+++ b/tests/cases/compiler/typeReferenceDirectives10.ts
@@ -2,6 +2,7 @@
 // @declaration: true
 // @typeRoots: /types
 // @traceResolution: true
+// @currentDirectory: /
 
 // @filename: /ref.d.ts
 export interface $ { x }
diff --git a/tests/cases/compiler/typeReferenceDirectives11.ts b/tests/cases/compiler/typeReferenceDirectives11.ts
index 2d93e22bdcf..8b56459cd69 100644
--- a/tests/cases/compiler/typeReferenceDirectives11.ts
+++ b/tests/cases/compiler/typeReferenceDirectives11.ts
@@ -4,6 +4,7 @@
 // @traceResolution: true
 // @types: lib
 // @out: output.js
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 
diff --git a/tests/cases/compiler/typeReferenceDirectives12.ts b/tests/cases/compiler/typeReferenceDirectives12.ts
index efdb1e8312b..f1abe27f05f 100644
--- a/tests/cases/compiler/typeReferenceDirectives12.ts
+++ b/tests/cases/compiler/typeReferenceDirectives12.ts
@@ -3,6 +3,7 @@
 // @typeRoots: /types
 // @traceResolution: true
 // @out: output.js
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 
diff --git a/tests/cases/compiler/typeReferenceDirectives13.ts b/tests/cases/compiler/typeReferenceDirectives13.ts
index 124c31274ac..f9dede73267 100644
--- a/tests/cases/compiler/typeReferenceDirectives13.ts
+++ b/tests/cases/compiler/typeReferenceDirectives13.ts
@@ -2,6 +2,7 @@
 // @declaration: true
 // @typeRoots: /types
 // @traceResolution: true
+// @currentDirectory: /
 
 // @filename: /ref.d.ts
 export interface $ { x }
diff --git a/tests/cases/compiler/typeReferenceDirectives2.ts b/tests/cases/compiler/typeReferenceDirectives2.ts
index 31a01a0b8e4..44218683a5a 100644
--- a/tests/cases/compiler/typeReferenceDirectives2.ts
+++ b/tests/cases/compiler/typeReferenceDirectives2.ts
@@ -3,6 +3,7 @@
 // @declaration: true
 // @typeRoots: /types
 // @types: lib
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 interface $ { x }
diff --git a/tests/cases/compiler/typeReferenceDirectives3.ts b/tests/cases/compiler/typeReferenceDirectives3.ts
index 4c2729ab389..1baf0bdac9d 100644
--- a/tests/cases/compiler/typeReferenceDirectives3.ts
+++ b/tests/cases/compiler/typeReferenceDirectives3.ts
@@ -2,6 +2,7 @@
 // @declaration: true
 // @typeRoots: /types
 // @traceResolution: true
+// @currentDirectory: /
 
 // $ comes from d.ts file - no need to add type reference directive
 
diff --git a/tests/cases/compiler/typeReferenceDirectives4.ts b/tests/cases/compiler/typeReferenceDirectives4.ts
index ac7346895ef..dfa87b65138 100644
--- a/tests/cases/compiler/typeReferenceDirectives4.ts
+++ b/tests/cases/compiler/typeReferenceDirectives4.ts
@@ -2,6 +2,7 @@
 // @traceResolution: true
 // @declaration: true
 // @typeRoots: /types
+// @currentDirectory: /
 
 // $ comes from d.ts file - no need to add type reference directive
 
diff --git a/tests/cases/compiler/typeReferenceDirectives5.ts b/tests/cases/compiler/typeReferenceDirectives5.ts
index bb24b82b324..e81ae663e24 100644
--- a/tests/cases/compiler/typeReferenceDirectives5.ts
+++ b/tests/cases/compiler/typeReferenceDirectives5.ts
@@ -2,6 +2,7 @@
 // @traceResolution: true
 // @declaration: true
 // @typeRoots: /types
+// @currentDirectory: /
 
 // @filename: /ref.d.ts
 export interface $ { x }
diff --git a/tests/cases/compiler/typeReferenceDirectives6.ts b/tests/cases/compiler/typeReferenceDirectives6.ts
index 7154963f1ef..edf2ece7e06 100644
--- a/tests/cases/compiler/typeReferenceDirectives6.ts
+++ b/tests/cases/compiler/typeReferenceDirectives6.ts
@@ -2,6 +2,7 @@
 // @traceResolution: true
 // @declaration: true
 // @typeRoots: /types
+// @currentDirectory: /
 
 // $ comes from type declaration file - type reference directive should be added
 
diff --git a/tests/cases/compiler/typeReferenceDirectives7.ts b/tests/cases/compiler/typeReferenceDirectives7.ts
index 79d42fa7018..e9335b07082 100644
--- a/tests/cases/compiler/typeReferenceDirectives7.ts
+++ b/tests/cases/compiler/typeReferenceDirectives7.ts
@@ -2,6 +2,7 @@
 // @traceResolution: true
 // @declaration: true
 // @typeRoots: /types
+// @currentDirectory: /
 
 // local value shadows global - no need to add type reference directive 
    
diff --git a/tests/cases/compiler/typeReferenceDirectives8.ts b/tests/cases/compiler/typeReferenceDirectives8.ts
index c7725a3aab1..bed69cbf357 100644
--- a/tests/cases/compiler/typeReferenceDirectives8.ts
+++ b/tests/cases/compiler/typeReferenceDirectives8.ts
@@ -3,6 +3,7 @@
 // @typeRoots: /types
 // @traceResolution: true
 // @types: lib
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 
diff --git a/tests/cases/compiler/typeReferenceDirectives9.ts b/tests/cases/compiler/typeReferenceDirectives9.ts
index 610c7173c89..1ad1aa52288 100644
--- a/tests/cases/compiler/typeReferenceDirectives9.ts
+++ b/tests/cases/compiler/typeReferenceDirectives9.ts
@@ -2,6 +2,7 @@
 // @declaration: true
 // @typeRoots: /types
 // @traceResolution: true
+// @currentDirectory: /
 
 // @filename: /types/lib/index.d.ts
 
diff --git a/tests/cases/compiler/unusedParameterProperty1.ts b/tests/cases/compiler/unusedParameterProperty1.ts
new file mode 100644
index 00000000000..61c4374c60a
--- /dev/null
+++ b/tests/cases/compiler/unusedParameterProperty1.ts
@@ -0,0 +1,9 @@
+//@noUnusedLocals:true
+//@noUnusedParameters:true
+
+class A {
+    constructor(private used: string) {
+        let foge = used;
+        foge += "";
+    }
+}
diff --git a/tests/cases/compiler/unusedParameterProperty2.ts b/tests/cases/compiler/unusedParameterProperty2.ts
new file mode 100644
index 00000000000..b9e05fbc967
--- /dev/null
+++ b/tests/cases/compiler/unusedParameterProperty2.ts
@@ -0,0 +1,9 @@
+//@noUnusedLocals:true
+//@noUnusedParameters:true
+
+class A {
+    constructor(private used) {
+        let foge = used;
+        foge += "";
+    }
+}
diff --git a/tests/cases/conformance/controlFlow/controlFlowIfStatement.ts b/tests/cases/conformance/controlFlow/controlFlowIfStatement.ts
index c9e9be92f8e..dc1cf97fe59 100644
--- a/tests/cases/conformance/controlFlow/controlFlowIfStatement.ts
+++ b/tests/cases/conformance/controlFlow/controlFlowIfStatement.ts
@@ -34,3 +34,19 @@ function b() {
     }
     x; // string
 }
+function c<T>(data: string | T): T {
+    if (typeof data === 'string') {
+        return JSON.parse(data);
+    }
+    else {
+        return data;
+    }
+}
+function d<T extends string>(data: string | T): never {
+    if (typeof data === 'string') {
+        throw new Error('will always happen');
+    }
+    else {
+        return data;
+    }
+}
diff --git a/tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts b/tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts
index b81dd26652b..0817954c35c 100644
--- a/tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts
+++ b/tests/cases/conformance/expressions/typeGuards/typeGuardsWithInstanceOfByConstructorSignature.ts
@@ -13,7 +13,7 @@ if (obj1 instanceof A) { // narrowed to A.
 }
 
 var obj2: any;
-if (obj2 instanceof A) { // can't narrow type from 'any'
+if (obj2 instanceof A) {
     obj2.foo;
     obj2.bar;
 }
@@ -35,7 +35,7 @@ if (obj3 instanceof B) { // narrowed to B<number>.
 }
 
 var obj4: any;
-if (obj4 instanceof B) { // can't narrow type from 'any'
+if (obj4 instanceof B) {
     obj4.foo = "str";
     obj4.foo = 1;
     obj4.bar = "str";
@@ -67,7 +67,7 @@ if (obj5 instanceof C) { // narrowed to C1|C2.
 }
 
 var obj6: any;
-if (obj6 instanceof C) { // can't narrow type from 'any'
+if (obj6 instanceof C) {
     obj6.foo;
     obj6.bar1;
     obj6.bar2;
@@ -86,7 +86,7 @@ if (obj7 instanceof D) { // narrowed to D.
 }
 
 var obj8: any;
-if (obj8 instanceof D) { // can't narrow type from 'any'
+if (obj8 instanceof D) {
     obj8.foo;
     obj8.bar;
 }
@@ -113,7 +113,7 @@ if (obj9 instanceof E) { // narrowed to E1 | E2
 }
 
 var obj10: any;
-if (obj10 instanceof E) { // can't narrow type from 'any'
+if (obj10 instanceof E) {
     obj10.foo;
     obj10.bar1;
     obj10.bar2;
@@ -136,7 +136,7 @@ if (obj11 instanceof F) { // can't type narrowing, construct signature returns a
 }
 
 var obj12: any;
-if (obj12 instanceof F) { // can't narrow type from 'any'
+if (obj12 instanceof F) {
     obj12.foo;
     obj12.bar;
 }
@@ -161,7 +161,7 @@ if (obj13 instanceof G) { // narrowed to G1. G1 is return type of prototype prop
 }
 
 var obj14: any;
-if (obj14 instanceof G) { // can't narrow type from 'any'
+if (obj14 instanceof G) {
     obj14.foo1;
     obj14.foo2;
 }
@@ -183,7 +183,19 @@ if (obj15 instanceof H) { // narrowed to H.
 }
 
 var obj16: any;
-if (obj16 instanceof H) { // can't narrow type from 'any'
+if (obj16 instanceof H) {
     obj16.foo1;
     obj16.foo2;
 }
+
+var obj17: any;
+if (obj17 instanceof Object) { // can't narrow type from 'any' to 'Object'
+    obj17.foo1;
+    obj17.foo2;
+}
+
+var obj18: any;
+if (obj18 instanceof Function) { // can't narrow type from 'any' to 'Function'
+    obj18.foo1;
+    obj18.foo2;
+}
diff --git a/tests/cases/conformance/jsx/tsxAttributeResolution14.tsx b/tests/cases/conformance/jsx/tsxAttributeResolution14.tsx
new file mode 100644
index 00000000000..1e4418e7fba
--- /dev/null
+++ b/tests/cases/conformance/jsx/tsxAttributeResolution14.tsx
@@ -0,0 +1,32 @@
+//@jsx: preserve
+//@module: amd
+
+//@filename: react.d.ts
+declare module JSX {
+	interface Element { }
+	interface IntrinsicElements {
+        div: any;
+	}
+    interface ElementAttributesProperty { prop: any }
+}
+
+//@filename: file.tsx
+
+interface IProps {
+  primaryText: string,
+  [propName: string]: string | number
+}
+
+function VerticalNavMenuItem(prop: IProps) {
+  return <div>props.primaryText</div>
+}
+
+function VerticalNav() {
+  return (
+    <div>
+      <VerticalNavMenuItem primaryText={2} />  // error
+      <VerticalNavMenuItem justRandomProp={2} primaryText={"hello"} />  // ok
+      <VerticalNavMenuItem justRandomProp1={true} primaryText={"hello"} />  // error
+    </div>
+  )
+} 
\ No newline at end of file
diff --git a/tests/cases/conformance/jsx/tsxAttributeResolution5.tsx b/tests/cases/conformance/jsx/tsxAttributeResolution5.tsx
index dd16ade10e3..83fb7b32f56 100644
--- a/tests/cases/conformance/jsx/tsxAttributeResolution5.tsx
+++ b/tests/cases/conformance/jsx/tsxAttributeResolution5.tsx
@@ -29,4 +29,4 @@ function make3<T extends {y: string}> (obj: T) {
 
 
 <test1 {...{}} />; // Error, missing x
-<test2 {...{}} />; // OK
+<test2 {...{}} />; // Error, missing toString
diff --git a/tests/cases/conformance/references/library-reference-13.ts b/tests/cases/conformance/references/library-reference-13.ts
index 92b4b259ba4..419643d9d0d 100644
--- a/tests/cases/conformance/references/library-reference-13.ts
+++ b/tests/cases/conformance/references/library-reference-13.ts
@@ -1,5 +1,6 @@
 // @noImplicitReferences: true
 // @traceResolution: true
+// @currentDirectory: /
 
 // load type declarations from types section of tsconfig
 
diff --git a/tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts b/tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts
new file mode 100644
index 00000000000..dfa60a415f9
--- /dev/null
+++ b/tests/cases/conformance/types/any/narrowExceptionVariableInCatchClause.ts
@@ -0,0 +1,23 @@
+declare function isFooError(x: any): x is { type: 'foo'; dontPanic(); };
+
+function tryCatch() {
+    try {
+        // do stuff...
+    }
+    catch (err) { // err is implicitly 'any' and cannot be annotated
+
+        if (isFooError(err)) {
+            err.dontPanic(); // OK
+            err.doPanic(); // ERROR: Property 'doPanic' does not exist on type '{...}'
+        }
+
+        else if (err instanceof Error) {
+            err.message;
+            err.massage; // ERROR: Property 'massage' does not exist on type 'Error'
+        }
+
+        else {
+            throw err;
+        }
+    }
+}
diff --git a/tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts b/tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts
new file mode 100644
index 00000000000..4fbfc46060a
--- /dev/null
+++ b/tests/cases/conformance/types/any/narrowFromAnyWithInstanceof.ts
@@ -0,0 +1,23 @@
+declare var x: any;
+
+if (x instanceof Function) { // 'any' is not narrowed when target type is 'Function'
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+
+if (x instanceof Object) { // 'any' is not narrowed when target type is 'Object'
+    x.method();
+    x();
+}
+
+if (x instanceof Error) { // 'any' is narrowed to types other than 'Function'/'Object'
+    x.message;
+    x.mesage;
+}
+
+if (x instanceof Date) {
+    x.getDate();
+    x.getHuors();
+}
diff --git a/tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts b/tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts
new file mode 100644
index 00000000000..473bd349b5f
--- /dev/null
+++ b/tests/cases/conformance/types/any/narrowFromAnyWithTypePredicate.ts
@@ -0,0 +1,34 @@
+declare var x: any;
+declare function isFunction(x): x is Function;
+declare function isObject(x): x is Object;
+declare function isAnything(x): x is {};
+declare function isError(x): x is Error;
+declare function isDate(x): x is Date;
+
+
+if (isFunction(x)) { // 'any' is not narrowed when target type is 'Function'
+    x();
+    x(1, 2, 3);
+    x("hello!");
+    x.prop;
+}
+
+if (isObject(x)) { // 'any' is not narrowed when target type is 'Object'
+    x.method();
+    x();
+}
+
+if (isAnything(x)) { // 'any' is narrowed to types other than 'Function'/'Object' (including {})
+    x.method();
+    x();
+}
+
+if (isError(x)) { 
+    x.message;
+    x.mesage;
+}
+
+if (isDate(x)) {
+    x.getDate();
+    x.getHuors();
+}
diff --git a/tests/cases/conformance/typings/typingsLookup1.ts b/tests/cases/conformance/typings/typingsLookup1.ts
index 555d4569af3..150deef992a 100644
--- a/tests/cases/conformance/typings/typingsLookup1.ts
+++ b/tests/cases/conformance/typings/typingsLookup1.ts
@@ -1,5 +1,6 @@
 // @traceResolution: true
 // @noImplicitReferences: true
+// @currentDirectory: /
 
 // @filename: /tsconfig.json
 { "files": "a.ts" }
diff --git a/tests/cases/conformance/typings/typingsLookup2.ts b/tests/cases/conformance/typings/typingsLookup2.ts
new file mode 100644
index 00000000000..90e1e463f0a
--- /dev/null
+++ b/tests/cases/conformance/typings/typingsLookup2.ts
@@ -0,0 +1,13 @@
+// @traceResolution: true
+// @noImplicitReferences: true
+// @currentDirectory: /
+// This tests that an @types package with `"typings": null` is not automatically included.
+// (If it were, this test would break because there are no typings to be found.)
+
+// @filename: /tsconfig.json
+{}
+
+// @filename: /node_modules/@types/angular2/package.json
+{ "typings": null }
+
+// @filename: /a.ts
diff --git a/tests/cases/conformance/typings/typingsLookup3.ts b/tests/cases/conformance/typings/typingsLookup3.ts
new file mode 100644
index 00000000000..a0bb15476b9
--- /dev/null
+++ b/tests/cases/conformance/typings/typingsLookup3.ts
@@ -0,0 +1,14 @@
+// @traceResolution: true
+// @noImplicitReferences: true
+// @currentDirectory: /
+// This tests that `types` references are automatically lowercased.
+
+// @filename: /tsconfig.json
+{ "files": "a.ts" }
+
+// @filename: /node_modules/@types/jquery/index.d.ts
+declare var $: { x: any };
+
+// @filename: /a.ts
+/// <reference types="JqUeRy" />
+$.x;
diff --git a/tests/cases/fourslash/findAllRefsForUMDModuleAlias1.ts b/tests/cases/fourslash/findAllRefsForUMDModuleAlias1.ts
new file mode 100644
index 00000000000..4177e154532
--- /dev/null
+++ b/tests/cases/fourslash/findAllRefsForUMDModuleAlias1.ts
@@ -0,0 +1,13 @@
+/// <reference path='fourslash.ts' />
+
+// @Filename: 0.d.ts
+//// export function doThing(): string;
+//// export function doTheOtherThing(): void;
+
+//// export as namespace [|myLib|];
+
+// @Filename: 1.ts
+//// /// <reference path="0.d.ts" />
+//// [|myLib|].doThing();
+
+verify.rangesReferenceEachOther();
diff --git a/tests/cases/fourslash/javaScriptClass1.ts b/tests/cases/fourslash/javaScriptClass1.ts
new file mode 100644
index 00000000000..19f7a39b615
--- /dev/null
+++ b/tests/cases/fourslash/javaScriptClass1.ts
@@ -0,0 +1,31 @@
+///<reference path="fourslash.ts" />
+
+// Classes have their shape inferred from assignments
+// to properties of 'this' in the constructor
+
+// @allowNonTsExtensions: true
+// @Filename: Foo.js
+//// class Foo {
+////    constructor() {
+////        this.bar = 'world';
+////        this.thing = () => 0;
+////        this.union = 'foo';
+////        this.union = 100;
+////    }
+//// }
+//// var x = new Foo();
+//// x/**/
+
+
+goTo.marker();
+edit.insert('.');
+verify.completionListContains("bar", /*displayText*/ undefined, /*documentation*/ undefined, "property");
+verify.completionListContains("thing", /*displayText*/ undefined, /*documentation*/ undefined, "property");
+verify.completionListContains("union", /*displayText*/ undefined, /*documentation*/ undefined, "property");
+
+edit.insert('bar.');
+verify.completionListContains("substr", /*displayText*/ undefined, /*documentation*/ undefined, "method");
+edit.backspace('bar.'.length);
+
+edit.insert('union.');
+verify.completionListContains("toString", /*displayText*/ undefined, /*documentation*/ undefined, "method");
\ No newline at end of file
diff --git a/tests/cases/fourslash/javaScriptClass2.ts b/tests/cases/fourslash/javaScriptClass2.ts
new file mode 100644
index 00000000000..d6daa320c5a
--- /dev/null
+++ b/tests/cases/fourslash/javaScriptClass2.ts
@@ -0,0 +1,22 @@
+///<reference path="fourslash.ts" />
+
+// In an inferred class, we can rename successfully
+
+// @allowNonTsExtensions: true
+// @Filename: Foo.js
+//// class Foo {
+////    constructor() {
+////        this.[|union|] = 'foo';
+////        this./*1*/[|union|] = 100;
+////    }
+////    method() { return this./*2*/[|union|]; }
+//// }
+//// var x = new Foo();
+//// x./*3*/[|union|];
+
+goTo.marker('1');
+verify.renameLocations(/*findInStrings*/false, /*findInComments*/false);
+goTo.marker('2');
+verify.renameLocations(/*findInStrings*/false, /*findInComments*/false);
+goTo.marker('3');
+verify.renameLocations(/*findInStrings*/false, /*findInComments*/false);
diff --git a/tests/cases/fourslash/javaScriptClass3.ts b/tests/cases/fourslash/javaScriptClass3.ts
new file mode 100644
index 00000000000..47004d53b04
--- /dev/null
+++ b/tests/cases/fourslash/javaScriptClass3.ts
@@ -0,0 +1,24 @@
+///<reference path="fourslash.ts" />
+
+// In an inferred class, we can to-to-def successfully
+
+// @allowNonTsExtensions: true
+// @Filename: Foo.js
+//// class Foo {
+////    constructor() {
+////        /*dst1*/this.alpha = 10;
+////        /*dst2*/this.beta = 'gamma';
+////    }
+////    method() { return this.alpha; }
+//// }
+//// var x = new Foo();
+//// x.alpha/*src1*/;
+//// x.beta/*src2*/;
+
+goTo.marker('src1');
+goTo.definition();
+verify.caretAtMarker('dst1');
+
+goTo.marker('src2');
+goTo.definition();
+verify.caretAtMarker('dst2');
diff --git a/tests/cases/fourslash/javaScriptClass4.ts b/tests/cases/fourslash/javaScriptClass4.ts
new file mode 100644
index 00000000000..1148d5f320e
--- /dev/null
+++ b/tests/cases/fourslash/javaScriptClass4.ts
@@ -0,0 +1,22 @@
+///<reference path="fourslash.ts" />
+
+// Classes have their shape inferred from assignments
+// to properties of 'this' in the constructor
+
+// @allowNonTsExtensions: true
+// @Filename: Foo.js
+//// class Foo {
+////    constructor() {
+////        /**
+////          * @type {string}
+////        */
+////        this.baz = null;
+////    }
+//// }
+//// var x = new Foo();
+//// x/**/
+
+goTo.marker();
+edit.insert('.baz.');
+verify.completionListContains("substr", /*displayText*/ undefined, /*documentation*/ undefined, "method");
+
diff --git a/tests/cases/fourslash/localGetReferences.ts b/tests/cases/fourslash/localGetReferences.ts
index ada4947acde..b05346a366a 100644
--- a/tests/cases/fourslash/localGetReferences.ts
+++ b/tests/cases/fourslash/localGetReferences.ts
@@ -205,12 +205,13 @@ const rangesByText = test.rangesByText();
 for (const text in rangesByText) {
     const ranges = rangesByText[text];
     if (text === "globalVar") {
-        function isShadow(r) {
-            return r.marker && r.marker.data && r.marker.data.shadow;
-        }
         verify.rangesReferenceEachOther(ranges.filter(isShadow));
         verify.rangesReferenceEachOther(ranges.filter(r => !isShadow(r)));
     } else {
         verify.rangesReferenceEachOther(ranges);
     }
 }
+
+function isShadow(r) {
+    return r.marker && r.marker.data && r.marker.data.shadow;
+}
diff --git a/tests/cases/fourslash/quickInfoForUMDModuleAlias.ts b/tests/cases/fourslash/quickInfoForUMDModuleAlias.ts
new file mode 100644
index 00000000000..085ea33b874
--- /dev/null
+++ b/tests/cases/fourslash/quickInfoForUMDModuleAlias.ts
@@ -0,0 +1,17 @@
+/// <reference path='fourslash.ts' />
+
+// @Filename: 0.d.ts
+//// export function doThing(): string;
+//// export function doTheOtherThing(): void;
+
+//// export as namespace /*0*/myLib;
+
+// @Filename: 1.ts
+//// /// <reference path="0.d.ts" />
+//// /*1*/myLib.doThing();
+
+goTo.marker("0");
+verify.quickInfoIs("export namespace myLib");
+
+goTo.marker("1");
+verify.quickInfoIs("export namespace myLib");
diff --git a/tests/cases/fourslash/renameAlias.ts b/tests/cases/fourslash/renameAlias.ts
index e3f57ac7b41..e0408af656f 100644
--- a/tests/cases/fourslash/renameAlias.ts
+++ b/tests/cases/fourslash/renameAlias.ts
@@ -4,8 +4,8 @@
 ////import [|M|] = SomeModule;
 ////import C = [|M|].SomeClass;
 
-let ranges = test.ranges()
-for (let range of ranges) {
-    goTo.position(range.start);
-    verify.renameLocations(/*findInStrings*/ false, /*findInComments*/ false);
+let ranges = test.ranges()
+for (let range of ranges) {
+    goTo.position(range.start);
+    verify.renameLocations(/*findInStrings*/ false, /*findInComments*/ false);
 }
\ No newline at end of file
diff --git a/tests/cases/fourslash/renameUMDModuleAlias1.ts b/tests/cases/fourslash/renameUMDModuleAlias1.ts
new file mode 100644
index 00000000000..94aabcdbde5
--- /dev/null
+++ b/tests/cases/fourslash/renameUMDModuleAlias1.ts
@@ -0,0 +1,17 @@
+/// <reference path='fourslash.ts' />
+
+// @Filename: 0.d.ts
+//// export function doThing(): string;
+//// export function doTheOtherThing(): void;
+
+//// export as namespace [|myLib|];
+
+// @Filename: 1.ts
+//// /// <reference path="0.d.ts" />
+//// [|myLib|].doThing();
+
+const ranges = test.ranges()
+for (const range of ranges) {
+    goTo.position(range.start);
+    verify.renameLocations(/*findInStrings*/ false, /*findInComments*/ false);
+}
\ No newline at end of file
diff --git a/tests/cases/fourslash/renameUMDModuleAlias2.ts b/tests/cases/fourslash/renameUMDModuleAlias2.ts
new file mode 100644
index 00000000000..0daa4087b0d
--- /dev/null
+++ b/tests/cases/fourslash/renameUMDModuleAlias2.ts
@@ -0,0 +1,14 @@
+/// <reference path='fourslash.ts' />
+
+// @Filename: 0.d.ts
+//// export function doThing(): string;
+//// export function doTheOtherThing(): void;
+
+//// export as namespace /**/[|myLib|];
+
+// @Filename: 1.ts
+//// /// <reference path="0.d.ts" />
+//// myLib.doThing();
+
+goTo.marker();
+verify.renameInfoSucceeded("myLib");
\ No newline at end of file
diff --git a/tests/webTestServer.ts b/tests/webTestServer.ts
index ac5f046c1dd..4d2cb7d4672 100644
--- a/tests/webTestServer.ts
+++ b/tests/webTestServer.ts
@@ -117,19 +117,21 @@ function handleResolutionRequest(filePath: string, res: http.ServerResponse) {
     let resolvedPath = path.resolve(filePath, "");
     resolvedPath = resolvedPath.substring(resolvedPath.indexOf("tests"));
     resolvedPath = switchToForwardSlashes(resolvedPath);
-    send("success", res, resolvedPath);
-    return;
+    send(ResponseCode.Success, res, resolvedPath);
 }
 
-function send(result: "fail", res: http.ServerResponse, contents: string, contentType?: string): void;
-function send(result: "success", res: http.ServerResponse, contents: string, contentType?: string): void;
-function send(result: "unknown", res: http.ServerResponse, contents: string, contentType?: string): void;
-function send(result: string, res: http.ServerResponse, contents: string, contentType?: string): void
-function send(result: string, res: http.ServerResponse, contents: string, contentType = "binary"): void {
-    const responseCode = result === "success" ? 200 : result === "fail" ? 500 : result === "unknown" ? 404 : parseInt(result);
+const enum ResponseCode {
+    Success = 200,
+    BadRequest = 400,
+    NotFound = 404,
+    MethodNotAllowed = 405,
+    PayloadTooLarge = 413,
+    Fail = 500
+}
+
+function send(responseCode: number, res: http.ServerResponse, contents: string, contentType = "binary"): void {
     res.writeHead(responseCode, { "Content-Type": contentType });
     res.end(contents);
-    return;
 }
 
 // Reads the data from a post request and passes it to the given callback
@@ -142,7 +144,7 @@ function processPost(req: http.ServerRequest, res: http.ServerResponse, callback
             queryData += data;
             if (queryData.length > 1e8) {
                 queryData = "";
-                send("413", res, undefined);
+                send(ResponseCode.PayloadTooLarge, res, undefined);
                 console.log("ERROR: destroying connection");
                 req.connection.destroy();
             }
@@ -155,7 +157,7 @@ function processPost(req: http.ServerRequest, res: http.ServerResponse, callback
 
     }
     else {
-        send("405", res, undefined);
+        send(ResponseCode.MethodNotAllowed, res, undefined);
     }
 }
 
@@ -201,16 +203,16 @@ function handleRequestOperation(req: http.ServerRequest, res: http.ServerRespons
     switch (operation) {
         case RequestType.GetDir:
             const filesInFolder = dir(reqPath, "", { recursive: true });
-            send("success", res, filesInFolder.join(","));
+            send(ResponseCode.Success, res, filesInFolder.join(","));
             break;
         case RequestType.GetFile:
             fs.readFile(reqPath, (err, file) => {
                 const contentType = contentTypeForExtension(path.extname(reqPath));
                 if (err) {
-                    send("fail", res, err.message, contentType);
+                    send(ResponseCode.NotFound, res, err.message, contentType);
                 }
                 else {
-                    send("success", res, <any>file, contentType);
+                    send(ResponseCode.Success, res, <any>file, contentType);
                 }
             });
             break;
@@ -222,33 +224,33 @@ function handleRequestOperation(req: http.ServerRequest, res: http.ServerRespons
             processPost(req, res, (data) => {
                 writeFile(reqPath, data, { recursive: true });
             });
-            send("success", res, undefined);
+            send(ResponseCode.Success, res, undefined);
             break;
         case RequestType.WriteDir:
             fs.mkdirSync(reqPath);
-            send("success", res, undefined);
+            send(ResponseCode.Success, res, undefined);
             break;
         case RequestType.DeleteFile:
             if (fs.existsSync(reqPath)) {
                 fs.unlinkSync(reqPath);
             }
-            send("success", res, undefined);
+            send(ResponseCode.Success, res, undefined);
             break;
         case RequestType.DeleteDir:
             if (fs.existsSync(reqPath)) {
                 fs.rmdirSync(reqPath);
             }
-            send("success", res, undefined);
+            send(ResponseCode.Success, res, undefined);
             break;
         case RequestType.AppendFile:
             processPost(req, res, (data) => {
                 fs.appendFileSync(reqPath, data);
             });
-            send("success", res, undefined);
+            send(ResponseCode.Success, res, undefined);
             break;
         case RequestType.Unknown:
         default:
-            send("unknown", res, undefined);
+            send(ResponseCode.BadRequest, res, undefined);
             break;
     }
 
diff --git a/tslint.json b/tslint.json
index a46e58f8c29..f058eab3dd6 100644
--- a/tslint.json
+++ b/tslint.json
@@ -32,7 +32,7 @@
             "property-declaration": "nospace",
             "variable-declaration": "nospace"
         }],
-        "next-line": [true, 
+        "next-line": [true,
             "check-catch",
             "check-else"
         ],
@@ -44,7 +44,6 @@
         "boolean-trivia": true,
         "type-operator-spacing": true,
         "prefer-const": true,
-        "no-in-operator": true,
         "no-increment-decrement": true,
         "object-literal-surrounding-space": true,
         "no-type-assertion-whitespace": true