diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 5f4cf8442d6..e7b759ff64f 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -416,9 +416,8 @@ namespace ts {
                 }
             }
             else {
-                // Exported module members are given 2 symbols: A local symbol that is classified with an ExportValue,
-                // ExportType, or ExportContainer flag, and an associated export symbol with all the correct flags set
-                // on it. There are 2 main reasons:
+                // Exported module members are given 2 symbols: A local symbol that is classified with an ExportValue flag,
+                // and an associated export symbol with all the correct flags set on it. There are 2 main reasons:
                 //
                 //   1. We treat locals and exports of the same name as mutually exclusive within a container.
                 //      That means the binder will issue a Duplicate Identifier error if you mix locals and exports
@@ -438,10 +437,7 @@ namespace ts {
                     (node as JSDocTypedefTag).name.kind === SyntaxKind.Identifier &&
                     ((node as JSDocTypedefTag).name as Identifier).isInJSDocNamespace;
                 if ((!isAmbientModule(node) && (hasExportModifier || container.flags & NodeFlags.ExportContext)) || isJSDocTypedefInJSDocNamespace) {
-                    const exportKind =
-                        (symbolFlags & SymbolFlags.Value ? SymbolFlags.ExportValue : 0) |
-                        (symbolFlags & SymbolFlags.Type ? SymbolFlags.ExportType : 0) |
-                        (symbolFlags & SymbolFlags.Namespace ? SymbolFlags.ExportNamespace : 0);
+                    const exportKind = symbolFlags & SymbolFlags.Value ? SymbolFlags.ExportValue : 0;
                     const local = declareSymbol(container.locals, /*parent*/ undefined, node, exportKind, symbolExcludes);
                     local.exportSymbol = declareSymbol(container.symbol.exports, container.symbol, node, symbolFlags, symbolExcludes);
                     node.localSymbol = local;
@@ -2288,7 +2284,7 @@ namespace ts {
             // When we create a property via 'exports.foo = bar', the 'exports.foo' property access
             // expression is the declaration
             setCommonJsModuleIndicator(node);
-            declareSymbol(file.symbol.exports, file.symbol, <PropertyAccessExpression>node.left, SymbolFlags.Property | SymbolFlags.Export, SymbolFlags.None);
+            declareSymbol(file.symbol.exports, file.symbol, <PropertyAccessExpression>node.left, SymbolFlags.Property | SymbolFlags.ExportValue, SymbolFlags.None);
         }
 
         function isExportsOrModuleExportsOrAlias(node: Node): boolean {
@@ -2329,7 +2325,7 @@ namespace ts {
 
             // 'module.exports = expr' assignment
             setCommonJsModuleIndicator(node);
-            declareSymbol(file.symbol.exports, file.symbol, node, SymbolFlags.Property | SymbolFlags.Export | SymbolFlags.ValueModule, SymbolFlags.None);
+            declareSymbol(file.symbol.exports, file.symbol, node, SymbolFlags.Property | SymbolFlags.ExportValue | SymbolFlags.ValueModule, SymbolFlags.None);
         }
 
         function bindThisPropertyAssignment(node: BinaryExpression) {
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 5fac69156eb..6c5fcc36e4d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -2010,7 +2010,7 @@ namespace ts {
             }
 
             function getAccessibleSymbolChainFromSymbolTableWorker(symbols: SymbolTable, visitedSymbolTables: SymbolTable[]): Symbol[] {
-                if (contains(visitedSymbolTables, symbols)) {
+                if (contains<SymbolTable>(visitedSymbolTables, symbols)) {
                     return undefined;
                 }
                 visitedSymbolTables.push(symbols);
@@ -2285,21 +2285,15 @@ namespace ts {
         }
 
         function symbolToString(symbol: Symbol, enclosingDeclaration?: Node, meaning?: SymbolFlags): string {
-            const writer = getSingleLineStringWriter();
-            getSymbolDisplayBuilder().buildSymbolDisplay(symbol, writer, enclosingDeclaration, meaning);
-            const result = writer.string();
-            releaseStringWriter(writer);
-
-            return result;
+            return usingSingleLineStringWriter(writer => {
+                getSymbolDisplayBuilder().buildSymbolDisplay(symbol, writer, enclosingDeclaration, meaning);
+            });
         }
 
         function signatureToString(signature: Signature, enclosingDeclaration?: Node, flags?: TypeFormatFlags, kind?: SignatureKind): string {
-            const writer = getSingleLineStringWriter();
-            getSymbolDisplayBuilder().buildSignatureDisplay(signature, writer, enclosingDeclaration, flags, kind);
-            const result = writer.string();
-            releaseStringWriter(writer);
-
-            return result;
+            return usingSingleLineStringWriter(writer => {
+                getSymbolDisplayBuilder().buildSignatureDisplay(signature, writer, enclosingDeclaration, flags, kind);
+            });
         }
 
         function typeToString(type: Type, enclosingDeclaration?: Node, flags?: TypeFormatFlags): string {
@@ -2996,12 +2990,9 @@ namespace ts {
         }
 
         function typePredicateToString(typePredicate: TypePredicate, enclosingDeclaration?: Declaration, flags?: TypeFormatFlags): string {
-            const writer = getSingleLineStringWriter();
-            getSymbolDisplayBuilder().buildTypePredicateDisplay(typePredicate, writer, enclosingDeclaration, flags);
-            const result = writer.string();
-            releaseStringWriter(writer);
-
-            return result;
+            return usingSingleLineStringWriter(writer => {
+                getSymbolDisplayBuilder().buildTypePredicateDisplay(typePredicate, writer, enclosingDeclaration, flags);
+            });
         }
 
         function formatUnionTypes(types: Type[]): Type[] {
@@ -4239,7 +4230,7 @@ namespace ts {
                 return addOptionality(declaredType, /*optional*/ declaration.questionToken && includeOptionality);
             }
 
-            if ((noImplicitAny || declaration.flags & NodeFlags.JavaScriptFile) &&
+            if ((noImplicitAny || isInJavaScriptFile(declaration)) &&
                 declaration.kind === SyntaxKind.VariableDeclaration && !isBindingPattern(declaration.name) &&
                 !(getCombinedModifierFlags(declaration) & ModifierFlags.Export) && !isInAmbientContext(declaration)) {
                 // If --noImplicitAny is on or the declaration is in a Javascript file,
@@ -4493,7 +4484,7 @@ namespace ts {
                 if (declaration.kind === SyntaxKind.ExportAssignment) {
                     return links.type = checkExpression((<ExportAssignment>declaration).expression);
                 }
-                if (declaration.flags & NodeFlags.JavaScriptFile && declaration.kind === SyntaxKind.JSDocPropertyTag && (<JSDocPropertyTag>declaration).typeExpression) {
+                if (isInJavaScriptFile(declaration) && declaration.kind === SyntaxKind.JSDocPropertyTag && (<JSDocPropertyTag>declaration).typeExpression) {
                     return links.type = getTypeFromTypeNode((<JSDocPropertyTag>declaration).typeExpression.type);
                 }
                 // Handle variable, parameter or property
@@ -4552,7 +4543,7 @@ namespace ts {
                 const getter = getDeclarationOfKind<AccessorDeclaration>(symbol, SyntaxKind.GetAccessor);
                 const setter = getDeclarationOfKind<AccessorDeclaration>(symbol, SyntaxKind.SetAccessor);
 
-                if (getter && getter.flags & NodeFlags.JavaScriptFile) {
+                if (getter && isInJavaScriptFile(getter)) {
                     const jsDocType = getTypeForDeclarationFromJSDocComment(getter);
                     if (jsDocType) {
                         return links.type = jsDocType;
@@ -6206,7 +6197,7 @@ namespace ts {
         }
 
         function isJSDocOptionalParameter(node: ParameterDeclaration) {
-            if (node.flags & NodeFlags.JavaScriptFile) {
+            if (isInJavaScriptFile(node)) {
                 if (node.type && node.type.kind === SyntaxKind.JSDocOptionalType) {
                     return true;
                 }
@@ -6892,6 +6883,7 @@ namespace ts {
                     case "Null":
                         return nullType;
                     case "Object":
+                    case "object":
                         return anyType;
                     case "Function":
                     case "function":
@@ -11065,8 +11057,14 @@ namespace ts {
             if (!links.switchTypes) {
                 // If all case clauses specify expressions that have unit types, we return an array
                 // of those unit types. Otherwise we return an empty array.
-                const types = map(switchStatement.caseBlock.clauses, getTypeOfSwitchClause);
-                links.switchTypes = !contains(types, undefined) ? types : emptyArray;
+                links.switchTypes = [];
+                for (const clause of switchStatement.caseBlock.clauses) {
+                    const type = getTypeOfSwitchClause(clause);
+                    if (type === undefined) {
+                        return links.switchTypes = emptyArray;
+                    }
+                    links.switchTypes.push(type);
+                }
             }
             return links.switchTypes;
         }
@@ -11875,6 +11873,8 @@ namespace ts {
         }
 
         function getTypeOfSymbolAtLocation(symbol: Symbol, location: Node) {
+            symbol = symbol.exportSymbol || symbol;
+
             // If we have an identifier or a property access at the given location, if the location is
             // an dotted name expression, and if the location is not an assignment target, obtain the type
             // of the expression (which will reflect control flow analysis). If the expression indeed
@@ -18800,7 +18800,7 @@ namespace ts {
                 // local symbol is undefined => this declaration is non-exported.
                 // however symbol might contain other declarations that are exported
                 symbol = getSymbolOfNode(node);
-                if (!(symbol.flags & SymbolFlags.Export)) {
+                if (!symbol.exportSymbol) {
                     // this is a pure local symbol (all declarations are non-exported) - no need to check anything
                     return;
                 }
@@ -18811,11 +18811,9 @@ namespace ts {
                 return;
             }
 
-            // we use SymbolFlags.ExportValue, SymbolFlags.ExportType and SymbolFlags.ExportNamespace
-            // to denote disjoint declarationSpaces (without making new enum type).
-            let exportedDeclarationSpaces = SymbolFlags.None;
-            let nonExportedDeclarationSpaces = SymbolFlags.None;
-            let defaultExportedDeclarationSpaces = SymbolFlags.None;
+            let exportedDeclarationSpaces = DeclarationSpaces.None;
+            let nonExportedDeclarationSpaces = DeclarationSpaces.None;
+            let defaultExportedDeclarationSpaces = DeclarationSpaces.None;
             for (const d of symbol.declarations) {
                 const declarationSpaces = getDeclarationSpaces(d);
                 const effectiveDeclarationFlags = getEffectiveDeclarationFlags(d, ModifierFlags.Export | ModifierFlags.Default);
@@ -18855,24 +18853,36 @@ namespace ts {
                 }
             }
 
-            function getDeclarationSpaces(d: Declaration): SymbolFlags {
+            const enum DeclarationSpaces {
+                None = 0,
+                ExportValue = 1 << 0,
+                ExportType = 1 << 1,
+                ExportNamespace = 1 << 2,
+            }
+            function getDeclarationSpaces(d: Declaration): DeclarationSpaces {
                 switch (d.kind) {
                     case SyntaxKind.InterfaceDeclaration:
-                        return SymbolFlags.ExportType;
+                    case SyntaxKind.TypeAliasDeclaration:
+                        return DeclarationSpaces.ExportType;
                     case SyntaxKind.ModuleDeclaration:
                         return isAmbientModule(d) || getModuleInstanceState(d) !== ModuleInstanceState.NonInstantiated
-                            ? SymbolFlags.ExportNamespace | SymbolFlags.ExportValue
-                            : SymbolFlags.ExportNamespace;
+                            ? DeclarationSpaces.ExportNamespace | DeclarationSpaces.ExportValue
+                            : DeclarationSpaces.ExportNamespace;
                     case SyntaxKind.ClassDeclaration:
                     case SyntaxKind.EnumDeclaration:
-                        return SymbolFlags.ExportType | SymbolFlags.ExportValue;
+                        return DeclarationSpaces.ExportType | DeclarationSpaces.ExportValue;
                     case SyntaxKind.ImportEqualsDeclaration:
-                        let result: SymbolFlags = 0;
+                        let result = DeclarationSpaces.None;
                         const target = resolveAlias(getSymbolOfNode(d));
                         forEach(target.declarations, d => { result |= getDeclarationSpaces(d); });
                         return result;
+                    case SyntaxKind.VariableDeclaration:
+                    case SyntaxKind.BindingElement:
+                    case SyntaxKind.FunctionDeclaration:
+                    case SyntaxKind.ImportSpecifier: // https://github.com/Microsoft/TypeScript/pull/7591
+                        return DeclarationSpaces.ExportValue;
                     default:
-                        return SymbolFlags.ExportValue;
+                        Debug.fail((ts as any).SyntaxKind[d.kind]);
                 }
             }
         }
@@ -20909,7 +20919,7 @@ namespace ts {
             }
         }
 
-        function areTypeParametersIdentical(declarations: (ClassDeclaration | InterfaceDeclaration)[], typeParameters: TypeParameter[]) {
+        function areTypeParametersIdentical(declarations: ReadonlyArray<ClassDeclaration | InterfaceDeclaration>, typeParameters: TypeParameter[]) {
             const maxTypeArgumentCount = length(typeParameters);
             const minTypeArgumentCount = getMinTypeArgumentCount(typeParameters);
 
@@ -22281,11 +22291,6 @@ namespace ts {
                     }
 
                     switch (location.kind) {
-                        case SyntaxKind.SourceFile:
-                            if (!isExternalOrCommonJsModule(<SourceFile>location)) {
-                                break;
-                            }
-                            // falls through
                         case SyntaxKind.ModuleDeclaration:
                             copySymbols(getSymbolOfNode(location).exports, meaning & SymbolFlags.ModuleMember);
                             break;
@@ -22337,7 +22342,7 @@ namespace ts {
              * @param meaning meaning of symbol to filter by before adding to symbol table
              */
             function copySymbol(symbol: Symbol, meaning: SymbolFlags): void {
-                if (symbol.flags & meaning) {
+                if (getCombinedLocalAndExportSymbolFlags(symbol) & meaning) {
                     const id = symbol.name;
                     // We will copy all symbol regardless of its reserved name because
                     // symbolsToArray will check whether the key is a reserved name and
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 94b6540f16a..159dc27bb47 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -105,7 +105,7 @@ namespace ts {
             paramType: Diagnostics.KIND,
             showInSimplifiedHelpView: true,
             category: Diagnostics.Basic_Options,
-            description: Diagnostics.Specify_module_code_generation_Colon_commonjs_amd_system_umd_es2015_or_ESNext,
+            description: Diagnostics.Specify_module_code_generation_Colon_none_commonjs_amd_system_umd_es2015_or_ESNext,
         },
         {
             name: "lib",
@@ -2162,7 +2162,7 @@ namespace ts {
      * @param extensionPriority The priority of the extension.
      * @param context The expansion context.
      */
-    function hasFileWithHigherPriorityExtension(file: string, literalFiles: Map<string>, wildcardFiles: Map<string>, extensions: string[], keyMapper: (value: string) => string) {
+    function hasFileWithHigherPriorityExtension(file: string, literalFiles: Map<string>, wildcardFiles: Map<string>, extensions: ReadonlyArray<string>, keyMapper: (value: string) => string) {
         const extensionPriority = getExtensionPriority(file, extensions);
         const adjustedExtensionPriority = adjustExtensionPriority(extensionPriority, extensions);
         for (let i = ExtensionPriority.Highest; i < adjustedExtensionPriority; i++) {
@@ -2184,7 +2184,7 @@ namespace ts {
      * @param extensionPriority The priority of the extension.
      * @param context The expansion context.
      */
-    function removeWildcardFilesWithLowerPriorityExtension(file: string, wildcardFiles: Map<string>, extensions: string[], keyMapper: (value: string) => string) {
+    function removeWildcardFilesWithLowerPriorityExtension(file: string, wildcardFiles: Map<string>, extensions: ReadonlyArray<string>, keyMapper: (value: string) => string) {
         const extensionPriority = getExtensionPriority(file, extensions);
         const nextExtensionPriority = getNextLowestExtensionPriority(extensionPriority, extensions);
         for (let i = nextExtensionPriority; i < extensions.length; i++) {
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 05466c3cc4b..a1880b46471 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -53,7 +53,7 @@ namespace ts {
     }
 
     /* @internal */
-    export function createSymbolTable(symbols?: Symbol[]): SymbolTable {
+    export function createSymbolTable(symbols?: ReadonlyArray<Symbol>): SymbolTable {
         const result = createMap<Symbol>() as SymbolTable;
         if (symbols) {
             for (const symbol of symbols) {
@@ -88,7 +88,7 @@ namespace ts {
 
         class MapIterator<T, U extends (string | T | [string, T])> {
             private data: MapLike<T>;
-            private keys: string[];
+            private keys: ReadonlyArray<string>;
             private index = 0;
             private selector: (data: MapLike<T>, key: string) => U;
             constructor(data: MapLike<T>, selector: (data: MapLike<T>, key: string) => U) {
@@ -175,7 +175,7 @@ namespace ts {
         GreaterThan = 1
     }
 
-    export function length(array: any[]) {
+    export function length(array: ReadonlyArray<any>) {
         return array ? array.length : 0;
     }
 
@@ -184,7 +184,7 @@ 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[] | undefined, callback: (element: T, index: number) => U | undefined): U | undefined {
+    export function forEach<T, U>(array: ReadonlyArray<T> | undefined, callback: (element: T, index: number) => U | undefined): U | undefined {
         if (array) {
             for (let i = 0; i < array.length; i++) {
                 const result = callback(array[i], i);
@@ -217,14 +217,14 @@ namespace ts {
         return undefined;
     }
 
-    export function zipWith<T, U>(arrayA: T[], arrayB: U[], callback: (a: T, b: U, index: number) => void): void {
+    export function zipWith<T, U>(arrayA: ReadonlyArray<T>, arrayB: ReadonlyArray<U>, callback: (a: T, b: U, index: number) => void): void {
         Debug.assert(arrayA.length === arrayB.length);
         for (let i = 0; i < arrayA.length; i++) {
             callback(arrayA[i], arrayB[i], i);
         }
     }
 
-    export function zipToMap<T>(keys: string[], values: T[]): Map<T> {
+    export function zipToMap<T>(keys: ReadonlyArray<string>, values: ReadonlyArray<T>): Map<T> {
         Debug.assert(keys.length === values.length);
         const map = createMap<T>();
         for (let i = 0; i < keys.length; ++i) {
@@ -238,7 +238,7 @@ namespace ts {
      * returns a falsey value, then returns false.
      * If no such value is found, the callback is applied to each element of array and `true` is returned.
      */
-    export function every<T>(array: T[], callback: (element: T, index: number) => boolean): boolean {
+    export function every<T>(array: ReadonlyArray<T>, callback: (element: T, index: number) => boolean): boolean {
         if (array) {
             for (let i = 0; i < array.length; i++) {
                 if (!callback(array[i], i)) {
@@ -251,7 +251,7 @@ namespace ts {
     }
 
     /** Works like Array.prototype.find, returning `undefined` if no element satisfying the predicate is found. */
-    export function find<T>(array: T[], predicate: (element: T, index: number) => boolean): T | undefined {
+    export function find<T>(array: ReadonlyArray<T>, predicate: (element: T, index: number) => boolean): T | undefined {
         for (let i = 0; i < array.length; i++) {
             const value = array[i];
             if (predicate(value, i)) {
@@ -262,7 +262,7 @@ namespace ts {
     }
 
     /** Works like Array.prototype.findIndex, returning `-1` if no element satisfying the predicate is found. */
-    export function findIndex<T>(array: T[], predicate: (element: T, index: number) => boolean): number {
+    export function findIndex<T>(array: ReadonlyArray<T>, predicate: (element: T, index: number) => boolean): number {
         for (let i = 0; i < array.length; i++) {
             if (predicate(array[i], i)) {
                 return i;
@@ -275,7 +275,7 @@ namespace ts {
      * Returns the first truthy result of `callback`, or else fails.
      * This is like `forEach`, but never returns undefined.
      */
-    export function findMap<T, U>(array: T[], callback: (element: T, index: number) => U | undefined): U {
+    export function findMap<T, U>(array: ReadonlyArray<T>, callback: (element: T, index: number) => U | undefined): U {
         for (let i = 0; i < array.length; i++) {
             const result = callback(array[i], i);
             if (result) {
@@ -285,7 +285,7 @@ namespace ts {
         Debug.fail();
     }
 
-    export function contains<T>(array: T[], value: T): boolean {
+    export function contains<T>(array: ReadonlyArray<T>, value: T): boolean {
         if (array) {
             for (const v of array) {
                 if (v === value) {
@@ -296,7 +296,7 @@ namespace ts {
         return false;
     }
 
-    export function indexOf<T>(array: T[], value: T): number {
+    export function indexOf<T>(array: ReadonlyArray<T>, value: T): number {
         if (array) {
             for (let i = 0; i < array.length; i++) {
                 if (array[i] === value) {
@@ -307,7 +307,7 @@ namespace ts {
         return -1;
     }
 
-    export function indexOfAnyCharCode(text: string, charCodes: number[], start?: number): number {
+    export function indexOfAnyCharCode(text: string, charCodes: ReadonlyArray<number>, start?: number): number {
         for (let i = start || 0; i < text.length; i++) {
             if (contains(charCodes, text.charCodeAt(i))) {
                 return i;
@@ -316,7 +316,7 @@ namespace ts {
         return -1;
     }
 
-    export function countWhere<T>(array: T[], predicate: (x: T, i: number) => boolean): number {
+    export function countWhere<T>(array: ReadonlyArray<T>, predicate: (x: T, i: number) => boolean): number {
         let count = 0;
         if (array) {
             for (let i = 0; i < array.length; i++) {
@@ -335,6 +335,8 @@ namespace ts {
      */
     export function filter<T, U extends T>(array: T[], f: (x: T) => x is U): U[];
     export function filter<T>(array: T[], f: (x: T) => boolean): T[];
+    export function filter<T, U extends T>(array: ReadonlyArray<T>, f: (x: T) => x is U): ReadonlyArray<U>;
+    export function filter<T, U extends T>(array: ReadonlyArray<T>, f: (x: T) => boolean): ReadonlyArray<T>;
     export function filter<T>(array: T[], f: (x: T) => boolean): T[] {
         if (array) {
             const len = array.length;
@@ -382,7 +384,7 @@ namespace ts {
         array.length = outIndex;
     }
 
-    export function map<T, U>(array: T[], f: (x: T, i: number) => U): U[] {
+    export function map<T, U>(array: ReadonlyArray<T>, f: (x: T, i: number) => U): U[] {
         let result: U[];
         if (array) {
             result = [];
@@ -394,6 +396,8 @@ namespace ts {
     }
 
     // Maps from T to T and avoids allocation if all elements map to themselves
+    export function sameMap<T>(array: T[], f: (x: T, i: number) => T): T[];
+    export function sameMap<T>(array: ReadonlyArray<T>, f: (x: T, i: number) => T): ReadonlyArray<T>;
     export function sameMap<T>(array: T[], f: (x: T, i: number) => T): T[] {
         let result: T[];
         if (array) {
@@ -419,7 +423,7 @@ namespace ts {
      *
      * @param array The array to flatten.
      */
-    export function flatten<T>(array: (T | T[])[]): T[] {
+    export function flatten<T>(array: ReadonlyArray<T | ReadonlyArray<T>>): T[] {
         let result: T[];
         if (array) {
             result = [];
@@ -444,7 +448,7 @@ namespace ts {
      * @param array The array to map.
      * @param mapfn The callback used to map the result into one or more values.
      */
-    export function flatMap<T, U>(array: T[] | undefined, mapfn: (x: T, i: number) => U | U[] | undefined): U[] | undefined {
+    export function flatMap<T, U>(array: ReadonlyArray<T> | undefined, mapfn: (x: T, i: number) => U | ReadonlyArray<U> | undefined): U[] | undefined {
         let result: U[];
         if (array) {
             result = [];
@@ -463,6 +467,17 @@ namespace ts {
         return result;
     }
 
+    export function flatMapIter<T, U>(iter: Iterator<T>, mapfn: (x: T) => U[] | undefined): U[] {
+        const result: U[] = [];
+        while (true) {
+            const { value, done } = iter.next();
+            if (done) break;
+            const res = mapfn(value);
+            if (res) result.push(...res);
+        }
+        return result;
+    }
+
     /**
      * Maps an array. If the mapped value is an array, it is spread into the result.
      * Avoids allocation if all elements map to themselves.
@@ -470,6 +485,8 @@ namespace ts {
      * @param array The array to map.
      * @param mapfn The callback used to map the result into one or more values.
      */
+    export function sameFlatMap<T>(array: T[], mapfn: (x: T, i: number) => T | ReadonlyArray<T>): T[];
+    export function sameFlatMap<T>(array: ReadonlyArray<T>, mapfn: (x: T, i: number) => T | ReadonlyArray<T>): ReadonlyArray<T>;
     export function sameFlatMap<T>(array: T[], mapfn: (x: T, i: number) => T | T[]): T[] {
         let result: T[];
         if (array) {
@@ -508,7 +525,7 @@ namespace ts {
      * Computes the first matching span of elements and returns a tuple of the first span
      * and the remaining elements.
      */
-    export function span<T>(array: T[], f: (x: T, i: number) => boolean): [T[], T[]] {
+    export function span<T>(array: ReadonlyArray<T>, f: (x: T, i: number) => boolean): [T[], T[]] {
         if (array) {
             for (let i = 0; i < array.length; i++) {
                 if (!f(array[i], i)) {
@@ -528,7 +545,7 @@ namespace ts {
      * @param keyfn A callback used to select the key for an element.
      * @param mapfn A callback used to map a contiguous chunk of values to a single value.
      */
-    export function spanMap<T, K, U>(array: T[], keyfn: (x: T, i: number) => K, mapfn: (chunk: T[], key: K, start: number, end: number) => U): U[] {
+    export function spanMap<T, K, U>(array: ReadonlyArray<T>, keyfn: (x: T, i: number) => K, mapfn: (chunk: T[], key: K, start: number, end: number) => U): U[] {
         let result: U[];
         if (array) {
             result = [];
@@ -581,7 +598,7 @@ namespace ts {
         return result;
     }
 
-    export function some<T>(array: T[], predicate?: (value: T) => boolean): boolean {
+    export function some<T>(array: ReadonlyArray<T>, predicate?: (value: T) => boolean): boolean {
         if (array) {
             if (predicate) {
                 for (const v of array) {
@@ -597,6 +614,8 @@ namespace ts {
         return false;
     }
 
+    export function concatenate<T>(array1: T[], array2: T[]): T[];
+    export function concatenate<T>(array1: ReadonlyArray<T>, array2: ReadonlyArray<T>): ReadonlyArray<T>;
     export function concatenate<T>(array1: T[], array2: T[]): T[] {
         if (!some(array2)) return array1;
         if (!some(array1)) return array2;
@@ -604,7 +623,7 @@ namespace ts {
     }
 
     // TODO: fixme (N^2) - add optional comparer so collection can be sorted before deduplication.
-    export function deduplicate<T>(array: T[], areEqual?: (a: T, b: T) => boolean): T[] {
+    export function deduplicate<T>(array: ReadonlyArray<T>, areEqual?: (a: T, b: T) => boolean): T[] {
         let result: T[];
         if (array) {
             result = [];
@@ -661,6 +680,8 @@ namespace ts {
     /**
      * Compacts an array, removing any falsey elements.
      */
+    export function compact<T>(array: T[]): T[];
+    export function compact<T>(array: ReadonlyArray<T>): ReadonlyArray<T>;
     export function compact<T>(array: T[]): T[] {
         let result: T[];
         if (array) {
@@ -700,10 +721,11 @@ namespace ts {
         return result;
     }
 
-    export function sum(array: any[], prop: string): number {
+    export function sum<T extends Record<K, number>, K extends string>(array: T[], prop: K): number {
         let result = 0;
         for (const v of array) {
-            result += v[prop];
+            // Note: we need the following type assertion because of GH #17069
+            result += v[prop] as number;
         }
         return result;
     }
@@ -727,7 +749,7 @@ namespace ts {
      * Gets the actual offset into an array for a relative offset. Negative offsets indicate a
      * position offset from the end of the array.
      */
-    function toOffset(array: any[], offset: number) {
+    function toOffset(array: ReadonlyArray<any>, offset: number) {
         return offset < 0 ? array.length + offset : offset;
     }
 
@@ -741,7 +763,7 @@ namespace ts {
      * @param start The offset in `from` at which to start copying values.
      * @param end The offset in `from` at which to stop copying values (non-inclusive).
      */
-    export function addRange<T>(to: T[] | undefined, from: T[] | undefined, start?: number, end?: number): T[] | undefined {
+    export function addRange<T>(to: T[] | undefined, from: ReadonlyArray<T> | undefined, start?: number, end?: number): T[] | undefined {
         if (from === undefined) return to;
         if (to === undefined) return from.slice(start, end);
         start = start === undefined ? 0 : toOffset(from, start);
@@ -758,14 +780,14 @@ namespace ts {
     /**
      * Stable sort of an array. Elements equal to each other maintain their relative position in the array.
      */
-    export function stableSort<T>(array: T[], comparer: (x: T, y: T) => Comparison = compareValues) {
+    export function stableSort<T>(array: ReadonlyArray<T>, comparer: (x: T, y: T) => Comparison = compareValues) {
         return array
             .map((_, i) => i) // create array of indices
             .sort((x, y) => comparer(array[x], array[y]) || compareValues(x, y)) // sort indices by value then position
             .map(i => array[i]); // get sorted array
     }
 
-    export function rangeEquals<T>(array1: T[], array2: T[], pos: number, end: number) {
+    export function rangeEquals<T>(array1: ReadonlyArray<T>, array2: ReadonlyArray<T>, pos: number, end: number) {
         while (pos < end) {
             if (array1[pos] !== array2[pos]) {
                 return false;
@@ -779,7 +801,7 @@ namespace ts {
      * Returns the element at a specific offset in an array if non-empty, `undefined` otherwise.
      * A negative offset indicates the element should be retrieved from the end of the array.
      */
-    export function elementAt<T>(array: T[] | undefined, offset: number): T | undefined {
+    export function elementAt<T>(array: ReadonlyArray<T> | undefined, offset: number): T | undefined {
         if (array) {
             offset = toOffset(array, offset);
             if (offset < array.length) {
@@ -792,21 +814,21 @@ namespace ts {
     /**
      * Returns the first element of an array if non-empty, `undefined` otherwise.
      */
-    export function firstOrUndefined<T>(array: T[]): T | undefined {
+    export function firstOrUndefined<T>(array: ReadonlyArray<T>): T | undefined {
         return elementAt(array, 0);
     }
 
     /**
      * Returns the last element of an array if non-empty, `undefined` otherwise.
      */
-    export function lastOrUndefined<T>(array: T[]): T | undefined {
+    export function lastOrUndefined<T>(array: ReadonlyArray<T>): T | undefined {
         return elementAt(array, -1);
     }
 
     /**
      * Returns the only element of an array if it contains only one element, `undefined` otherwise.
      */
-    export function singleOrUndefined<T>(array: T[]): T | undefined {
+    export function singleOrUndefined<T>(array: ReadonlyArray<T>): T | undefined {
         return array && array.length === 1
             ? array[0]
             : undefined;
@@ -816,13 +838,15 @@ namespace ts {
      * Returns the only element of an array if it contains only one element; otheriwse, returns the
      * array.
      */
+    export function singleOrMany<T>(array: T[]): T | T[];
+    export function singleOrMany<T>(array: ReadonlyArray<T>): T | ReadonlyArray<T>;
     export function singleOrMany<T>(array: T[]): T | T[] {
         return array && array.length === 1
             ? array[0]
             : array;
     }
 
-    export function replaceElement<T>(array: T[], index: number, value: T): T[] {
+    export function replaceElement<T>(array: ReadonlyArray<T>, index: number, value: T): T[] {
         const result = array.slice(0);
         result[index] = value;
         return result;
@@ -835,7 +859,7 @@ namespace ts {
      * @param array A sorted array whose first element must be no larger than number
      * @param number The value to be searched for in the array.
      */
-    export function binarySearch<T>(array: T[], value: T, comparer?: (v1: T, v2: T) => number, offset?: number): number {
+    export function binarySearch<T>(array: ReadonlyArray<T>, value: T, comparer?: (v1: T, v2: T) => number, offset?: number): number {
         if (!array || array.length === 0) {
             return -1;
         }
@@ -864,8 +888,8 @@ namespace ts {
         return ~low;
     }
 
-    export function reduceLeft<T, U>(array: T[], f: (memo: U, value: T, i: number) => U, initial: U, start?: number, count?: number): U;
-    export function reduceLeft<T>(array: T[], f: (memo: T, value: T, i: number) => T): T;
+    export function reduceLeft<T, U>(array: ReadonlyArray<T>, f: (memo: U, value: T, i: number) => U, initial: U, start?: number, count?: number): U;
+    export function reduceLeft<T>(array: ReadonlyArray<T>, f: (memo: T, value: T, i: number) => T): T;
     export function reduceLeft<T>(array: T[], f: (memo: T, value: T, i: number) => T, initial?: T, start?: number, count?: number): T {
         if (array && array.length > 0) {
             const size = array.length;
@@ -890,8 +914,8 @@ namespace ts {
         return initial;
     }
 
-    export function reduceRight<T, U>(array: T[], f: (memo: U, value: T, i: number) => U, initial: U, start?: number, count?: number): U;
-    export function reduceRight<T>(array: T[], f: (memo: T, value: T, i: number) => T): T;
+    export function reduceRight<T, U>(array: ReadonlyArray<T>, f: (memo: U, value: T, i: number) => U, initial: U, start?: number, count?: number): U;
+    export function reduceRight<T>(array: ReadonlyArray<T>, f: (memo: T, value: T, i: number) => T): T;
     export function reduceRight<T>(array: T[], f: (memo: T, value: T, i: number) => T, initial?: T, start?: number, count?: number): T {
         if (array) {
             const size = array.length;
@@ -1058,9 +1082,9 @@ 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>;
-    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> {
+    export function arrayToMap<T>(array: ReadonlyArray<T>, makeKey: (value: T) => string): Map<T>;
+    export function arrayToMap<T, U>(array: ReadonlyArray<T>, makeKey: (value: T) => string, makeValue: (value: T) => U): Map<U>;
+    export function arrayToMap<T, U>(array: ReadonlyArray<T>, makeKey: (value: T) => string, makeValue?: (value: T) => U): Map<T | U> {
         const result = createMap<T | U>();
         for (const value of array) {
             result.set(makeKey(value), makeValue ? makeValue(value) : value);
@@ -1073,9 +1097,9 @@ namespace ts {
      *
      * @param array the array of input elements.
      */
-    export function arrayToSet(array: string[]): Map<true>;
-    export function arrayToSet<T>(array: T[], makeKey: (value: T) => string): Map<true>;
-    export function arrayToSet(array: any[], makeKey?: (value: any) => string): Map<true> {
+    export function arrayToSet(array: ReadonlyArray<string>): Map<true>;
+    export function arrayToSet<T>(array: ReadonlyArray<T>, makeKey: (value: T) => string): Map<true>;
+    export function arrayToSet(array: ReadonlyArray<any>, makeKey?: (value: any) => string): Map<true> {
         return arrayToMap<any, true>(array, makeKey || (s => s), () => true);
     }
 
@@ -1158,7 +1182,7 @@ namespace ts {
     /**
      * Tests whether a value is an array.
      */
-    export function isArray(value: any): value is any[] {
+    export function isArray(value: any): value is ReadonlyArray<any> {
         return Array.isArray ? Array.isArray(value) : value instanceof Array;
     }
 
@@ -1628,7 +1652,7 @@ namespace ts {
         return getNormalizedPathFromPathComponents(getNormalizedPathComponents(fileName, currentDirectory));
     }
 
-    export function getNormalizedPathFromPathComponents(pathComponents: string[]) {
+    export function getNormalizedPathFromPathComponents(pathComponents: ReadonlyArray<string>) {
         if (pathComponents && pathComponents.length) {
             return pathComponents[0] + pathComponents.slice(1).join(directorySeparator);
         }
@@ -1830,7 +1854,7 @@ namespace ts {
     }
 
     /* @internal */
-    export function fileExtensionIsOneOf(path: string, extensions: string[]): boolean {
+    export function fileExtensionIsOneOf(path: string, extensions: ReadonlyArray<string>): boolean {
         for (const extension of extensions) {
             if (fileExtensionIs(path, extension)) {
                 return true;
@@ -1855,7 +1879,7 @@ namespace ts {
     const singleAsteriskRegexFragmentFiles = "([^./]|(\\.(?!min\\.js$))?)*";
     const singleAsteriskRegexFragmentOther = "[^/]*";
 
-    export function getRegularExpressionForWildcard(specs: string[], basePath: string, usage: "files" | "directories" | "exclude"): string | undefined {
+    export function getRegularExpressionForWildcard(specs: ReadonlyArray<string>, basePath: string, usage: "files" | "directories" | "exclude"): string | undefined {
         const patterns = getRegularExpressionsForWildcards(specs, basePath, usage);
         if (!patterns || !patterns.length) {
             return undefined;
@@ -1867,7 +1891,7 @@ namespace ts {
         return `^(${pattern})${terminator}`;
     }
 
-    function getRegularExpressionsForWildcards(specs: string[], basePath: string, usage: "files" | "directories" | "exclude"): string[] | undefined {
+    function getRegularExpressionsForWildcards(specs: ReadonlyArray<string>, basePath: string, usage: "files" | "directories" | "exclude"): string[] | undefined {
         if (specs === undefined || specs.length === 0) {
             return undefined;
         }
@@ -1972,21 +1996,21 @@ namespace ts {
     }
 
     export interface FileSystemEntries {
-        files: string[];
-        directories: string[];
+        files: ReadonlyArray<string>;
+        directories: ReadonlyArray<string>;
     }
 
     export interface FileMatcherPatterns {
         /** One pattern for each "include" spec. */
-        includeFilePatterns: string[];
+        includeFilePatterns: ReadonlyArray<string>;
         /** One pattern matching one of any of the "include" specs. */
         includeFilePattern: string;
         includeDirectoryPattern: string;
         excludePattern: string;
-        basePaths: string[];
+        basePaths: ReadonlyArray<string>;
     }
 
-    export function getFileMatcherPatterns(path: string, excludes: string[], includes: string[], useCaseSensitiveFileNames: boolean, currentDirectory: string): FileMatcherPatterns {
+    export function getFileMatcherPatterns(path: string, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, useCaseSensitiveFileNames: boolean, currentDirectory: string): FileMatcherPatterns {
         path = normalizePath(path);
         currentDirectory = normalizePath(currentDirectory);
         const absolutePath = combinePaths(currentDirectory, path);
@@ -2000,7 +2024,7 @@ namespace ts {
         };
     }
 
-    export function matchFiles(path: string, extensions: string[], excludes: string[], includes: string[], useCaseSensitiveFileNames: boolean, currentDirectory: string, depth: number | undefined, getFileSystemEntries: (path: string) => FileSystemEntries): string[] {
+    export function matchFiles(path: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, useCaseSensitiveFileNames: boolean, currentDirectory: string, depth: number | undefined, getFileSystemEntries: (path: string) => FileSystemEntries): string[] {
         path = normalizePath(path);
         currentDirectory = normalizePath(currentDirectory);
 
@@ -2020,7 +2044,7 @@ namespace ts {
             visitDirectory(basePath, combinePaths(currentDirectory, basePath), depth);
         }
 
-        return flatten(results);
+        return flatten<string>(results);
 
         function visitDirectory(path: string, absolutePath: string, depth: number | undefined) {
             let { files, directories } = getFileSystemEntries(path);
@@ -2064,7 +2088,7 @@ namespace ts {
     /**
      * Computes the unique non-wildcard base paths amongst the provided include patterns.
      */
-    function getBasePaths(path: string, includes: string[], useCaseSensitiveFileNames: boolean) {
+    function getBasePaths(path: string, includes: ReadonlyArray<string>, useCaseSensitiveFileNames: boolean) {
         // Storage for our results in the form of literal paths (e.g. the paths as written by the user).
         const basePaths: string[] = [path];
 
@@ -2136,13 +2160,13 @@ namespace ts {
     /**
      *  List of supported extensions in order of file resolution precedence.
      */
-    export const supportedTypeScriptExtensions = [Extension.Ts, Extension.Tsx, Extension.Dts];
+    export const supportedTypeScriptExtensions: ReadonlyArray<Extension> = [Extension.Ts, Extension.Tsx, Extension.Dts];
     /** Must have ".d.ts" first because if ".ts" goes first, that will be detected as the extension instead of ".d.ts". */
-    export const supportedTypescriptExtensionsForExtractExtension = [Extension.Dts, Extension.Ts, Extension.Tsx];
-    export const supportedJavascriptExtensions = [Extension.Js, Extension.Jsx];
-    const allSupportedExtensions = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
+    export const supportedTypescriptExtensionsForExtractExtension: ReadonlyArray<Extension> = [Extension.Dts, Extension.Ts, Extension.Tsx];
+    export const supportedJavascriptExtensions: ReadonlyArray<Extension> = [Extension.Js, Extension.Jsx];
+    const allSupportedExtensions = [...supportedTypeScriptExtensions, ...supportedJavascriptExtensions];
 
-    export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: JsFileExtensionInfo[]): string[] {
+    export function getSupportedExtensions(options?: CompilerOptions, extraFileExtensions?: ReadonlyArray<JsFileExtensionInfo>): ReadonlyArray<string> {
         const needAllExtensions = options && options.allowJs;
         if (!extraFileExtensions || extraFileExtensions.length === 0 || !needAllExtensions) {
             return needAllExtensions ? allSupportedExtensions : supportedTypeScriptExtensions;
@@ -2164,7 +2188,7 @@ namespace ts {
         return forEach(supportedTypeScriptExtensions, extension => fileExtensionIs(fileName, extension));
     }
 
-    export function isSupportedSourceFileName(fileName: string, compilerOptions?: CompilerOptions, extraFileExtensions?: JsFileExtensionInfo[]) {
+    export function isSupportedSourceFileName(fileName: string, compilerOptions?: CompilerOptions, extraFileExtensions?: ReadonlyArray<JsFileExtensionInfo>) {
         if (!fileName) { return false; }
 
         for (const extension of getSupportedExtensions(compilerOptions, extraFileExtensions)) {
@@ -2188,7 +2212,7 @@ namespace ts {
         Lowest = DeclarationAndJavaScriptFiles,
     }
 
-    export function getExtensionPriority(path: string, supportedExtensions: string[]): ExtensionPriority {
+    export function getExtensionPriority(path: string, supportedExtensions: ReadonlyArray<string>): ExtensionPriority {
         for (let i = supportedExtensions.length - 1; i >= 0; i--) {
             if (fileExtensionIs(path, supportedExtensions[i])) {
                 return adjustExtensionPriority(<ExtensionPriority>i, supportedExtensions);
@@ -2203,7 +2227,7 @@ namespace ts {
     /**
      * Adjusts an extension priority to be the highest priority within the same range.
      */
-    export function adjustExtensionPriority(extensionPriority: ExtensionPriority, supportedExtensions: string[]): ExtensionPriority {
+    export function adjustExtensionPriority(extensionPriority: ExtensionPriority, supportedExtensions: ReadonlyArray<string>): ExtensionPriority {
         if (extensionPriority < ExtensionPriority.DeclarationAndJavaScriptFiles) {
             return ExtensionPriority.TypeScriptFiles;
         }
@@ -2212,12 +2236,13 @@ namespace ts {
         }
         else {
             return supportedExtensions.length;
-        }    }
+        }
+    }
 
     /**
      * Gets the next lowest extension priority for a given priority.
      */
-    export function getNextLowestExtensionPriority(extensionPriority: ExtensionPriority, supportedExtensions: string[]): ExtensionPriority {
+    export function getNextLowestExtensionPriority(extensionPriority: ExtensionPriority, supportedExtensions: ReadonlyArray<string>): ExtensionPriority {
         if (extensionPriority < ExtensionPriority.DeclarationAndJavaScriptFiles) {
             return ExtensionPriority.DeclarationAndJavaScriptFiles;
         }
@@ -2406,7 +2431,7 @@ namespace ts {
      * (These are verified by verifyCompilerOptions to have 0 or 1 "*" characters.)
      */
     /* @internal */
-    export function matchPatternOrExact(patternStrings: string[], candidate: string): string | Pattern | undefined {
+    export function matchPatternOrExact(patternStrings: ReadonlyArray<string>, candidate: string): string | Pattern | undefined {
         const patterns: Pattern[] = [];
         for (const patternString of patternStrings) {
             const pattern = tryParsePattern(patternString);
@@ -2439,7 +2464,7 @@ namespace ts {
 
     /** Return the object corresponding to the best pattern to match `candidate`. */
     /* @internal */
-    export function findBestPatternMatch<T>(values: T[], getPattern: (value: T) => Pattern, candidate: string): T | undefined {
+    export function findBestPatternMatch<T>(values: ReadonlyArray<T>, getPattern: (value: T) => Pattern, candidate: string): T | undefined {
         let matchedValue: T | undefined = undefined;
         // use length of prefix as betterness criteria
         let longestMatchPrefixLength = -1;
@@ -2495,7 +2520,7 @@ namespace ts {
         Debug.fail(`File ${path} has unknown extension.`);
     }
     export function tryGetExtensionFromPath(path: string): Extension | undefined {
-        return find(supportedTypescriptExtensionsForExtractExtension, e => fileExtensionIs(path, e)) || find(supportedJavascriptExtensions, e => fileExtensionIs(path, e));
+        return find<Extension>(supportedTypescriptExtensionsForExtractExtension, e => fileExtensionIs(path, e)) || find(supportedJavascriptExtensions, e => fileExtensionIs(path, e));
     }
 
     export function isCheckJsEnabledForFile(sourceFile: SourceFile, compilerOptions: CompilerOptions) {
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 1fd434abf4c..cf198ec06ec 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2658,7 +2658,7 @@
         "category": "Message",
         "code": 6015
     },
-    "Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'.": {
+    "Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'.": {
         "category": "Message",
         "code": 6016
     },
diff --git a/src/compiler/parser.ts b/src/compiler/parser.ts
index 4a979fb3e6f..089bff53a05 100644
--- a/src/compiler/parser.ts
+++ b/src/compiler/parser.ts
@@ -2,6 +2,15 @@
 /// <reference path="scanner.ts"/>
 
 namespace ts {
+    const enum SignatureFlags {
+        None = 0,
+        Yield = 1 << 0,
+        Await = 1 << 1,
+        Type  = 1 << 2,
+        RequireCompleteParameterList = 1 << 3,
+        IgnoreMissingOpenBrace = 1 << 4,
+    }
+
     let NodeConstructor: new (kind: SyntaxKind, pos: number, end: number) => Node;
     let TokenConstructor: new (kind: SyntaxKind, pos: number, end: number) => Node;
     let IdentifierConstructor: new (kind: SyntaxKind, pos: number, end: number) => Node;
@@ -2218,15 +2227,12 @@ namespace ts {
 
         function fillSignature(
             returnToken: SyntaxKind.ColonToken | SyntaxKind.EqualsGreaterThanToken,
-            yieldContext: boolean,
-            awaitContext: boolean,
-            requireCompleteParameterList: boolean,
+            flags: SignatureFlags,
             signature: SignatureDeclaration): void {
+            signature.typeParameters = parseTypeParameters();
+            signature.parameters = parseParameterList(flags);
 
             const returnTokenRequired = returnToken === SyntaxKind.EqualsGreaterThanToken;
-            signature.typeParameters = parseTypeParameters();
-            signature.parameters = parseParameterList(yieldContext, awaitContext, requireCompleteParameterList);
-
             if (returnTokenRequired) {
                 parseExpected(returnToken);
                 signature.type = parseTypeOrTypePredicate();
@@ -2234,9 +2240,19 @@ namespace ts {
             else if (parseOptional(returnToken)) {
                 signature.type = parseTypeOrTypePredicate();
             }
+            else if (flags & SignatureFlags.Type) {
+                const start = scanner.getTokenPos();
+                const length = scanner.getTextPos() - start;
+                const backwardToken = parseOptional(returnToken === SyntaxKind.ColonToken ? SyntaxKind.EqualsGreaterThanToken : SyntaxKind.ColonToken);
+                if (backwardToken) {
+                    // This is easy to get backward, especially in type contexts, so parse the type anyway
+                    signature.type = parseTypeOrTypePredicate();
+                    parseErrorAtPosition(start, length, Diagnostics._0_expected, tokenToString(returnToken));
+                }
+            }
         }
 
-        function parseParameterList(yieldContext: boolean, awaitContext: boolean, requireCompleteParameterList: boolean) {
+        function parseParameterList(flags: SignatureFlags) {
             // FormalParameters [Yield,Await]: (modified)
             //      [empty]
             //      FormalParameterList[?Yield,Await]
@@ -2254,15 +2270,15 @@ namespace ts {
                 const savedYieldContext = inYieldContext();
                 const savedAwaitContext = inAwaitContext();
 
-                setYieldContext(yieldContext);
-                setAwaitContext(awaitContext);
+                setYieldContext(!!(flags & SignatureFlags.Yield));
+                setAwaitContext(!!(flags & SignatureFlags.Await));
 
                 const result = parseDelimitedList(ParsingContext.Parameters, parseParameter);
 
                 setYieldContext(savedYieldContext);
                 setAwaitContext(savedAwaitContext);
 
-                if (!parseExpected(SyntaxKind.CloseParenToken) && requireCompleteParameterList) {
+                if (!parseExpected(SyntaxKind.CloseParenToken) && (flags & SignatureFlags.RequireCompleteParameterList)) {
                     // Caller insisted that we had to end with a )   We didn't.  So just return
                     // undefined here.
                     return undefined;
@@ -2274,7 +2290,7 @@ namespace ts {
             // We didn't even have an open paren.  If the caller requires a complete parameter list,
             // we definitely can't provide that.  However, if they're ok with an incomplete one,
             // then just return an empty set of parameters.
-            return requireCompleteParameterList ? undefined : createMissingList<ParameterDeclaration>();
+            return (flags & SignatureFlags.RequireCompleteParameterList) ? undefined : createMissingList<ParameterDeclaration>();
         }
 
         function parseTypeMemberSemicolon() {
@@ -2293,7 +2309,7 @@ namespace ts {
             if (kind === SyntaxKind.ConstructSignature) {
                 parseExpected(SyntaxKind.NewKeyword);
             }
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ false, /*awaitContext*/ false, /*requireCompleteParameterList*/ false, node);
+            fillSignature(SyntaxKind.ColonToken, SignatureFlags.Type, node);
             parseTypeMemberSemicolon();
             return addJSDocComment(finishNode(node));
         }
@@ -2383,7 +2399,7 @@ namespace ts {
 
                 // Method signatures don't exist in expression contexts.  So they have neither
                 // [Yield] nor [Await]
-                fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ false, /*awaitContext*/ false, /*requireCompleteParameterList*/ false, method);
+                fillSignature(SyntaxKind.ColonToken, SignatureFlags.Type, method);
                 parseTypeMemberSemicolon();
                 return addJSDocComment(finishNode(method));
             }
@@ -2527,7 +2543,7 @@ namespace ts {
             if (kind === SyntaxKind.ConstructorType) {
                 parseExpected(SyntaxKind.NewKeyword);
             }
-            fillSignature(SyntaxKind.EqualsGreaterThanToken, /*yieldContext*/ false, /*awaitContext*/ false, /*requireCompleteParameterList*/ false, node);
+            fillSignature(SyntaxKind.EqualsGreaterThanToken, SignatureFlags.Type, node);
             return finishNode(node);
         }
 
@@ -3254,7 +3270,7 @@ namespace ts {
         function parseParenthesizedArrowFunctionExpressionHead(allowAmbiguity: boolean): ArrowFunction {
             const node = <ArrowFunction>createNode(SyntaxKind.ArrowFunction);
             node.modifiers = parseModifiersForArrowFunction();
-            const isAsync = !!(getModifierFlags(node) & ModifierFlags.Async);
+            const isAsync = (getModifierFlags(node) & ModifierFlags.Async) ? SignatureFlags.Await : SignatureFlags.None;
 
             // Arrow functions are never generators.
             //
@@ -3263,7 +3279,7 @@ namespace ts {
             // a => (b => c)
             // And think that "(b =>" was actually a parenthesized arrow function with a missing
             // close paren.
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ false, /*awaitContext*/ isAsync, /*requireCompleteParameterList*/ !allowAmbiguity, node);
+            fillSignature(SyntaxKind.ColonToken, isAsync | (allowAmbiguity ? SignatureFlags.None : SignatureFlags.RequireCompleteParameterList), node);
 
             // If we couldn't get parameters, we definitely could not parse out an arrow function.
             if (!node.parameters) {
@@ -3288,7 +3304,7 @@ namespace ts {
 
         function parseArrowFunctionExpressionBody(isAsync: boolean): Block | Expression {
             if (token() === SyntaxKind.OpenBraceToken) {
-                return parseFunctionBlock(/*allowYield*/ false, /*allowAwait*/ isAsync, /*ignoreMissingOpenBrace*/ false);
+                return parseFunctionBlock(isAsync ? SignatureFlags.Await : SignatureFlags.None);
             }
 
             if (token() !== SyntaxKind.SemicolonToken &&
@@ -3309,8 +3325,8 @@ namespace ts {
                 // try to recover better.  If we don't do this, then the next close curly we see may end
                 // up preemptively closing the containing construct.
                 //
-                // Note: even when 'ignoreMissingOpenBrace' is passed as true, parseBody will still error.
-                return parseFunctionBlock(/*allowYield*/ false, /*allowAwait*/ isAsync, /*ignoreMissingOpenBrace*/ true);
+                // Note: even when 'IgnoreMissingOpenBrace' is passed, parseBody will still error.
+                return parseFunctionBlock(SignatureFlags.IgnoreMissingOpenBrace | (isAsync ? SignatureFlags.Await : SignatureFlags.None));
             }
 
             return isAsync
@@ -4386,16 +4402,16 @@ namespace ts {
             parseExpected(SyntaxKind.FunctionKeyword);
             node.asteriskToken = parseOptionalToken(SyntaxKind.AsteriskToken);
 
-            const isGenerator = !!node.asteriskToken;
-            const isAsync = !!(getModifierFlags(node) & ModifierFlags.Async);
+            const isGenerator = node.asteriskToken ? SignatureFlags.Yield : SignatureFlags.None;
+            const isAsync = (getModifierFlags(node) & ModifierFlags.Async) ? SignatureFlags.Await : SignatureFlags.None;
             node.name =
                 isGenerator && isAsync ? doInYieldAndAwaitContext(parseOptionalIdentifier) :
                     isGenerator ? doInYieldContext(parseOptionalIdentifier) :
                         isAsync ? doInAwaitContext(parseOptionalIdentifier) :
                             parseOptionalIdentifier();
 
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ isGenerator, /*awaitContext*/ isAsync, /*requireCompleteParameterList*/ false, node);
-            node.body = parseFunctionBlock(/*allowYield*/ isGenerator, /*allowAwait*/ isAsync, /*ignoreMissingOpenBrace*/ false);
+            fillSignature(SyntaxKind.ColonToken, isGenerator | isAsync, node);
+            node.body = parseFunctionBlock(isGenerator | isAsync);
 
             if (saveDecoratorContext) {
                 setDecoratorContext(/*val*/ true);
@@ -4444,12 +4460,12 @@ namespace ts {
             return finishNode(node);
         }
 
-        function parseFunctionBlock(allowYield: boolean, allowAwait: boolean, ignoreMissingOpenBrace: boolean, diagnosticMessage?: DiagnosticMessage): Block {
+        function parseFunctionBlock(flags: SignatureFlags, diagnosticMessage?: DiagnosticMessage): Block {
             const savedYieldContext = inYieldContext();
-            setYieldContext(allowYield);
+            setYieldContext(!!(flags & SignatureFlags.Yield));
 
             const savedAwaitContext = inAwaitContext();
-            setAwaitContext(allowAwait);
+            setAwaitContext(!!(flags & SignatureFlags.Await));
 
             // We may be in a [Decorator] context when parsing a function expression or
             // arrow function. The body of the function is not in [Decorator] context.
@@ -4458,7 +4474,7 @@ namespace ts {
                 setDecoratorContext(/*val*/ false);
             }
 
-            const block = parseBlock(ignoreMissingOpenBrace, diagnosticMessage);
+            const block = parseBlock(!!(flags & SignatureFlags.IgnoreMissingOpenBrace), diagnosticMessage);
 
             if (saveDecoratorContext) {
                 setDecoratorContext(/*val*/ true);
@@ -5005,13 +5021,13 @@ namespace ts {
             return !scanner.hasPrecedingLineBreak() && (isIdentifier() || token() === SyntaxKind.StringLiteral);
         }
 
-        function parseFunctionBlockOrSemicolon(isGenerator: boolean, isAsync: boolean, diagnosticMessage?: DiagnosticMessage): Block {
+        function parseFunctionBlockOrSemicolon(flags: SignatureFlags, diagnosticMessage?: DiagnosticMessage): Block {
             if (token() !== SyntaxKind.OpenBraceToken && canParseSemicolon()) {
                 parseSemicolon();
                 return;
             }
 
-            return parseFunctionBlock(isGenerator, isAsync, /*ignoreMissingOpenBrace*/ false, diagnosticMessage);
+            return parseFunctionBlock(flags, diagnosticMessage);
         }
 
         // DECLARATIONS
@@ -5146,10 +5162,10 @@ namespace ts {
             parseExpected(SyntaxKind.FunctionKeyword);
             node.asteriskToken = parseOptionalToken(SyntaxKind.AsteriskToken);
             node.name = hasModifier(node, ModifierFlags.Default) ? parseOptionalIdentifier() : parseIdentifier();
-            const isGenerator = !!node.asteriskToken;
-            const isAsync = hasModifier(node, ModifierFlags.Async);
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ isGenerator, /*awaitContext*/ isAsync, /*requireCompleteParameterList*/ false, node);
-            node.body = parseFunctionBlockOrSemicolon(isGenerator, isAsync, Diagnostics.or_expected);
+            const isGenerator = node.asteriskToken ? SignatureFlags.Yield : SignatureFlags.None;
+            const isAsync = hasModifier(node, ModifierFlags.Async) ? SignatureFlags.Await : SignatureFlags.None;
+            fillSignature(SyntaxKind.ColonToken, isGenerator | isAsync, node);
+            node.body = parseFunctionBlockOrSemicolon(isGenerator | isAsync, Diagnostics.or_expected);
             return addJSDocComment(finishNode(node));
         }
 
@@ -5158,8 +5174,8 @@ namespace ts {
             node.decorators = decorators;
             node.modifiers = modifiers;
             parseExpected(SyntaxKind.ConstructorKeyword);
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ false, /*awaitContext*/ false, /*requireCompleteParameterList*/ false, node);
-            node.body = parseFunctionBlockOrSemicolon(/*isGenerator*/ false, /*isAsync*/ false, Diagnostics.or_expected);
+            fillSignature(SyntaxKind.ColonToken, SignatureFlags.None, node);
+            node.body = parseFunctionBlockOrSemicolon(SignatureFlags.None, Diagnostics.or_expected);
             return addJSDocComment(finishNode(node));
         }
 
@@ -5170,10 +5186,10 @@ namespace ts {
             method.asteriskToken = asteriskToken;
             method.name = name;
             method.questionToken = questionToken;
-            const isGenerator = !!asteriskToken;
-            const isAsync = hasModifier(method, ModifierFlags.Async);
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ isGenerator, /*awaitContext*/ isAsync, /*requireCompleteParameterList*/ false, method);
-            method.body = parseFunctionBlockOrSemicolon(isGenerator, isAsync, diagnosticMessage);
+            const isGenerator = asteriskToken ? SignatureFlags.Yield : SignatureFlags.None;
+            const isAsync = hasModifier(method, ModifierFlags.Async) ? SignatureFlags.Await : SignatureFlags.None;
+            fillSignature(SyntaxKind.ColonToken, isGenerator | isAsync, method);
+            method.body = parseFunctionBlockOrSemicolon(isGenerator | isAsync, diagnosticMessage);
             return addJSDocComment(finishNode(method));
         }
 
@@ -5226,8 +5242,8 @@ namespace ts {
             node.decorators = decorators;
             node.modifiers = modifiers;
             node.name = parsePropertyName();
-            fillSignature(SyntaxKind.ColonToken, /*yieldContext*/ false, /*awaitContext*/ false, /*requireCompleteParameterList*/ false, node);
-            node.body = parseFunctionBlockOrSemicolon(/*isGenerator*/ false, /*isAsync*/ false);
+            fillSignature(SyntaxKind.ColonToken, SignatureFlags.None, node);
+            node.body = parseFunctionBlockOrSemicolon(SignatureFlags.None);
             return addJSDocComment(finishNode(node));
         }
 
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 97f6f36f3a1..153e4506ea9 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -447,7 +447,7 @@ namespace ts {
         let moduleResolutionCache: ModuleResolutionCache;
         let resolveModuleNamesWorker: (moduleNames: string[], containingFile: string) => ResolvedModuleFull[];
         if (host.resolveModuleNames) {
-            resolveModuleNamesWorker = (moduleNames, containingFile) => host.resolveModuleNames(moduleNames, containingFile).map(resolved => {
+            resolveModuleNamesWorker = (moduleNames, containingFile) => host.resolveModuleNames(checkAllDefined(moduleNames), containingFile).map(resolved => {
                 // An older host may have omitted extension, in which case we should infer it from the file extension of resolvedFileName.
                 if (!resolved || (resolved as ResolvedModuleFull).extension !== undefined) {
                     return resolved as ResolvedModuleFull;
@@ -460,16 +460,16 @@ namespace ts {
         else {
             moduleResolutionCache = createModuleResolutionCache(currentDirectory, x => host.getCanonicalFileName(x));
             const loader = (moduleName: string, containingFile: string) => resolveModuleName(moduleName, containingFile, options, host, moduleResolutionCache).resolvedModule;
-            resolveModuleNamesWorker = (moduleNames, containingFile) => loadWithLocalCache(moduleNames, containingFile, loader);
+            resolveModuleNamesWorker = (moduleNames, containingFile) => loadWithLocalCache(checkAllDefined(moduleNames), containingFile, loader);
         }
 
         let resolveTypeReferenceDirectiveNamesWorker: (typeDirectiveNames: string[], containingFile: string) => ResolvedTypeReferenceDirective[];
         if (host.resolveTypeReferenceDirectives) {
-            resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile) => host.resolveTypeReferenceDirectives(typeDirectiveNames, containingFile);
+            resolveTypeReferenceDirectiveNamesWorker = (typeDirectiveNames, containingFile) => host.resolveTypeReferenceDirectives(checkAllDefined(typeDirectiveNames), containingFile);
         }
         else {
             const loader = (typesRef: string, containingFile: string) => resolveTypeReferenceDirective(typesRef, containingFile, options, host).resolvedTypeReferenceDirective;
-            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(typeReferenceDirectiveNames, containingFile, loader);
+            resolveTypeReferenceDirectiveNamesWorker = (typeReferenceDirectiveNames, containingFile) => loadWithLocalCache(checkAllDefined(typeReferenceDirectiveNames), containingFile, loader);
         }
 
         const filesByName = createMap<SourceFile | undefined>();
@@ -2127,4 +2127,9 @@ namespace ts {
             return options.allowJs ? undefined : Diagnostics.Module_0_was_resolved_to_1_but_allowJs_is_not_set;
         }
     }
+
+    function checkAllDefined(names: string[]): string[] {
+        Debug.assert(names.every(name => name !== undefined), "A name is undefined.", () => JSON.stringify(names));
+        return names;
+    }
 }
diff --git a/src/compiler/scanner.ts b/src/compiler/scanner.ts
index dd43b3c8263..16555ff7937 100644
--- a/src/compiler/scanner.ts
+++ b/src/compiler/scanner.ts
@@ -326,13 +326,20 @@ namespace ts {
     }
 
     export function getPositionOfLineAndCharacter(sourceFile: SourceFile, line: number, character: number): number {
-        return computePositionOfLineAndCharacter(getLineStarts(sourceFile), line, character);
+        return computePositionOfLineAndCharacter(getLineStarts(sourceFile), line, character, sourceFile.text);
     }
 
     /* @internal */
-    export function computePositionOfLineAndCharacter(lineStarts: number[], line: number, character: number): number {
+    export function computePositionOfLineAndCharacter(lineStarts: number[], line: number, character: number, debugText?: string): number {
         Debug.assert(line >= 0 && line < lineStarts.length);
-        return lineStarts[line] + character;
+        const res = lineStarts[line] + character;
+        if (line < lineStarts.length - 1) {
+            Debug.assert(res < lineStarts[line + 1]);
+        }
+        else if (debugText !== undefined) {
+            Debug.assert(res < debugText.length);
+        }
+        return res;
     }
 
     /* @internal */
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index d5102da0b3f..3e3ff90c00c 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -39,7 +39,7 @@ namespace ts {
         getExecutingFilePath(): string;
         getCurrentDirectory(): string;
         getDirectories(path: string): string[];
-        readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[];
+        readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
         getModifiedTime?(path: string): Date;
         /**
          * This should be cryptographically secure.
@@ -100,7 +100,7 @@ namespace ts {
         readFile(path: string): string;
         writeFile(path: string, contents: string): void;
         getDirectories(path: string): string[];
-        readDirectory(path: string, extensions?: string[], basePaths?: string[], excludeEx?: string, includeFileEx?: string, includeDirEx?: string): string[];
+        readDirectory(path: string, extensions?: ReadonlyArray<string>, basePaths?: ReadonlyArray<string>, excludeEx?: string, includeFileEx?: string, includeDirEx?: string): string[];
         watchFile?(path: string, callback: FileWatcherCallback): FileWatcher;
         watchDirectory?(path: string, callback: DirectoryWatcherCallback, recursive?: boolean): FileWatcher;
         realpath(path: string): string;
@@ -287,7 +287,7 @@ namespace ts {
                 }
             }
 
-            function readDirectory(path: string, extensions?: string[], excludes?: string[], includes?: string[], depth?: number): string[] {
+            function readDirectory(path: string, extensions?: ReadonlyArray<string>, excludes?: ReadonlyArray<string>, includes?: ReadonlyArray<string>, depth?: number): string[] {
                 return matchFiles(path, extensions, excludes, includes, useCaseSensitiveFileNames, process.cwd(), depth, getAccessibleFileSystemEntries);
             }
 
diff --git a/src/compiler/transformers/es2015.ts b/src/compiler/transformers/es2015.ts
index 3de73440193..19f9d00c656 100644
--- a/src/compiler/transformers/es2015.ts
+++ b/src/compiler/transformers/es2015.ts
@@ -3579,7 +3579,7 @@ namespace ts {
             // Map spans of spread expressions into their expressions and spans of other
             // expressions into an array literal.
             const numElements = elements.length;
-            const segments = flatten(
+            const segments = flatten<Expression>(
                 spanMap(elements, partitionSpread, (partition, visitPartition, _start, end) =>
                     visitPartition(partition, multiLine, hasTrailingComma && end === numElements)
                 )
diff --git a/src/compiler/transformers/jsx.ts b/src/compiler/transformers/jsx.ts
index eab5f69bd4f..ca22cb701d0 100644
--- a/src/compiler/transformers/jsx.ts
+++ b/src/compiler/transformers/jsx.ts
@@ -88,7 +88,7 @@ namespace ts {
             else {
                 // Map spans of JsxAttribute nodes into object literals and spans
                 // of JsxSpreadAttribute nodes into expressions.
-                const segments = flatten(
+                const segments = flatten<Expression | ObjectLiteralExpression>(
                     spanMap(attrs, isJsxSpreadAttribute, (attrs, isSpread) => isSpread
                         ? map(attrs, transformJsxSpreadAttributeToExpression)
                         : createObjectLiteral(map(attrs, transformJsxAttributeToObjectLiteralElement))
diff --git a/src/compiler/transformers/ts.ts b/src/compiler/transformers/ts.ts
index 5d6efa489b5..5fd1b7b62da 100644
--- a/src/compiler/transformers/ts.ts
+++ b/src/compiler/transformers/ts.ts
@@ -1377,7 +1377,7 @@ namespace ts {
 
             const decoratorExpressions: Expression[] = [];
             addRange(decoratorExpressions, map(allDecorators.decorators, transformDecorator));
-            addRange(decoratorExpressions, flatMap(allDecorators.parameters, transformDecoratorsOfParameter));
+            addRange(decoratorExpressions, flatMap<Decorator[], Expression>(allDecorators.parameters, transformDecoratorsOfParameter));
             addTypeMetadata(node, container, decoratorExpressions);
             return decoratorExpressions;
         }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 88bfff872d5..c26fa5603ea 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2406,7 +2406,7 @@ namespace ts {
     export interface ParseConfigHost {
         useCaseSensitiveFileNames: boolean;
 
-        readDirectory(rootDir: string, extensions: string[], excludes: string[], includes: string[], depth: number): string[];
+        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, depth: number): string[];
 
         /**
          * Gets a value indicating whether the specified path exists and is a file.
@@ -2890,13 +2890,11 @@ namespace ts {
         TypeParameter           = 1 << 18,  // Type parameter
         TypeAlias               = 1 << 19,  // Type alias
         ExportValue             = 1 << 20,  // Exported value marker (see comment in declareModuleMember in binder)
-        ExportType              = 1 << 21,  // Exported type marker (see comment in declareModuleMember in binder)
-        ExportNamespace         = 1 << 22,  // Exported namespace marker (see comment in declareModuleMember in binder)
-        Alias                   = 1 << 23,  // An alias for another symbol (see comment in isAliasSymbolDeclaration in checker)
-        Prototype               = 1 << 24,  // Prototype property (no source representation)
-        ExportStar              = 1 << 25,  // Export * declaration
-        Optional                = 1 << 26,  // Optional property
-        Transient               = 1 << 27,  // Transient symbol (created during type check)
+        Alias                   = 1 << 21,  // An alias for another symbol (see comment in isAliasSymbolDeclaration in checker)
+        Prototype               = 1 << 22,  // Prototype property (no source representation)
+        ExportStar              = 1 << 23,  // Export * declaration
+        Optional                = 1 << 24,  // Optional property
+        Transient               = 1 << 25,  // Transient symbol (created during type check)
 
         Enum = RegularEnum | ConstEnum,
         Variable = FunctionScopedVariable | BlockScopedVariable,
@@ -2941,7 +2939,6 @@ namespace ts {
         BlockScoped = BlockScopedVariable | Class | Enum,
 
         PropertyOrAccessor = Property | Accessor,
-        Export = ExportNamespace | ExportType | ExportValue,
 
         ClassMember = Method | Accessor | Property,
 
@@ -4354,7 +4351,7 @@ namespace ts {
      */
     export type Visitor = (node: Node) => VisitResult<Node>;
 
-    export type VisitResult<T extends Node> = T | T[];
+    export type VisitResult<T extends Node> = T | T[] | undefined;
 
     export interface Printer {
         /**
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 04479e6f9cd..c8751f497a6 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -44,42 +44,47 @@ namespace ts {
         string(): string;
     }
 
-    // Pool writers to avoid needing to allocate them for every symbol we write.
-    const stringWriters: StringSymbolWriter[] = [];
-    export function getSingleLineStringWriter(): StringSymbolWriter {
-        if (stringWriters.length === 0) {
-            let str = "";
+    const stringWriter = createSingleLineStringWriter();
+    let stringWriterAcquired = false;
 
-            const writeText: (text: string) => void = text => str += text;
-            return {
-                string: () => str,
-                writeKeyword: writeText,
-                writeOperator: writeText,
-                writePunctuation: writeText,
-                writeSpace: writeText,
-                writeStringLiteral: writeText,
-                writeParameter: writeText,
-                writeProperty: writeText,
-                writeSymbol: writeText,
+    function createSingleLineStringWriter(): StringSymbolWriter {
+        let str = "";
 
-                // Completely ignore indentation for string writers.  And map newlines to
-                // a single space.
-                writeLine: () => str += " ",
-                increaseIndent: noop,
-                decreaseIndent: noop,
-                clear: () => str = "",
-                trackSymbol: noop,
-                reportInaccessibleThisError: noop,
-                reportPrivateInBaseOfClassExpression: noop,
-            };
-        }
+        const writeText: (text: string) => void = text => str += text;
+        return {
+            string: () => str,
+            writeKeyword: writeText,
+            writeOperator: writeText,
+            writePunctuation: writeText,
+            writeSpace: writeText,
+            writeStringLiteral: writeText,
+            writeParameter: writeText,
+            writeProperty: writeText,
+            writeSymbol: writeText,
 
-        return stringWriters.pop();
+            // Completely ignore indentation for string writers.  And map newlines to
+            // a single space.
+            writeLine: () => str += " ",
+            increaseIndent: noop,
+            decreaseIndent: noop,
+            clear: () => str = "",
+            trackSymbol: noop,
+            reportInaccessibleThisError: noop,
+            reportPrivateInBaseOfClassExpression: noop,
+        };
     }
 
-    export function releaseStringWriter(writer: StringSymbolWriter) {
-        writer.clear();
-        stringWriters.push(writer);
+    export function usingSingleLineStringWriter(action: (writer: StringSymbolWriter) => void): string {
+        try {
+            Debug.assert(!stringWriterAcquired);
+            stringWriterAcquired = true;
+            action(stringWriter);
+            return stringWriter.string();
+        }
+        finally {
+            stringWriter.clear();
+            stringWriterAcquired = false;
+        }
     }
 
     export function getFullWidth(node: Node) {
@@ -1619,7 +1624,7 @@ namespace ts {
     }
 
     export function isRestParameter(node: ParameterDeclaration) {
-        if (node && (node.flags & NodeFlags.JavaScriptFile)) {
+        if (isInJavaScriptFile(node)) {
             if (node.type && node.type.kind === SyntaxKind.JSDocVariadicType ||
                 forEach(getJSDocParameterTags(node),
                         t => t.typeExpression && t.typeExpression.type.kind === SyntaxKind.JSDocVariadicType)) {
@@ -2743,7 +2748,7 @@ namespace ts {
         if (node.typeParameters) {
             return node.typeParameters;
         }
-        if (node.flags & NodeFlags.JavaScriptFile) {
+        if (isInJavaScriptFile(node)) {
             const templateTag = getJSDocTemplateTag(node);
             return templateTag && templateTag.typeParameters;
         }
@@ -3334,7 +3339,7 @@ namespace ts {
             }
         }
 
-        return stableSort(result, (x, y) => compareValues(x[0], y[0]));
+        return stableSort<[number, string]>(result, (x, y) => compareValues(x[0], y[0]));
     }
 
     export function formatSyntaxKind(kind: SyntaxKind): string {
@@ -3598,6 +3603,15 @@ namespace ts {
         }
         return previous[previous.length - 1];
     }
+
+    export function skipAlias(symbol: Symbol, checker: TypeChecker) {
+        return symbol.flags & SymbolFlags.Alias ? checker.getAliasedSymbol(symbol) : symbol;
+    }
+
+    /** See comment on `declareModuleMember` in `binder.ts`. */
+    export function getCombinedLocalAndExportSymbolFlags(symbol: Symbol): SymbolFlags {
+        return symbol.exportSymbol ? symbol.exportSymbol.flags | symbol.flags : symbol.flags;
+    }
 }
 
 namespace ts {
@@ -5116,6 +5130,7 @@ namespace ts {
             || kind === SyntaxKind.ThisKeyword
             || kind === SyntaxKind.TrueKeyword
             || kind === SyntaxKind.SuperKeyword
+            || kind === SyntaxKind.ImportKeyword
             || kind === SyntaxKind.NonNullExpression
             || kind === SyntaxKind.MetaProperty;
     }
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index 434a3981461..5e83c5339c6 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -219,7 +219,7 @@ namespace FourSlash {
 
         // Add input file which has matched file name with the given reference-file path.
         // This is necessary when resolveReference flag is specified
-        private addMatchedInputFile(referenceFilePath: string, extensions: string[]) {
+        private addMatchedInputFile(referenceFilePath: string, extensions: ReadonlyArray<string>) {
             const inputFiles = this.inputFiles;
             const languageServiceAdapterHost = this.languageServiceAdapterHost;
             if (!extensions) {
@@ -605,7 +605,7 @@ namespace FourSlash {
                 this.verifyGoToXPlain(arg0, endMarkerNames, getDefs);
             }
             else if (ts.isArray(arg0)) {
-                const pairs: [string | string[], string | string[]][] = arg0;
+                const pairs: ReadonlyArray<[string | string[], string | string[]]> = arg0;
                 for (const [start, end] of pairs) {
                     this.verifyGoToXPlain(start, end, getDefs);
                 }
@@ -949,6 +949,22 @@ namespace FourSlash {
             this.verifySymbol(symbol, declarationRanges);
         }
 
+        public symbolsInScope(range: Range): ts.Symbol[] {
+            const node = this.goToAndGetNode(range);
+            return this.getChecker().getSymbolsInScope(node, ts.SymbolFlags.Value | ts.SymbolFlags.Type | ts.SymbolFlags.Namespace);
+        }
+
+        public verifyTypeOfSymbolAtLocation(range: Range, symbol: ts.Symbol, expected: string): void {
+            const node = this.goToAndGetNode(range);
+            const checker = this.getChecker();
+            const type = checker.getTypeOfSymbolAtLocation(symbol, node);
+
+            const actual = checker.typeToString(type);
+            if (actual !== expected) {
+                this.raiseError(`Expected: '${expected}', actual: '${actual}'`);
+            }
+        }
+
         private verifyReferencesAre(expectedReferences: Range[]) {
             const actualReferences = this.getReferencesAtCaret() || [];
 
@@ -1790,7 +1806,14 @@ namespace FourSlash {
                 this.editScriptAndUpdateMarkers(fileName, offsetStart, offsetEnd, edit.newText);
                 const editDelta = edit.newText.length - edit.span.length;
                 if (offsetStart <= this.currentCaretPosition) {
-                    this.currentCaretPosition += editDelta;
+                    if (offsetEnd <= this.currentCaretPosition) {
+                        // The entirety of the edit span falls before the caret position, shift the caret accordingly
+                        this.currentCaretPosition += editDelta;
+                    }
+                    else {
+                        // The span being replaced includes the caret position, place the caret at the beginning of the span
+                        this.currentCaretPosition = offsetStart;
+                    }
                 }
                 runningOffset += editDelta;
                 // TODO: Consider doing this at least some of the time for higher fidelity. Currently causes a failure (bug 707150)
@@ -2320,35 +2343,25 @@ namespace FourSlash {
          * @param fileName Path to file where error should be retrieved from.
          */
         private getCodeFixActions(fileName: string, errorCode?: number): ts.CodeAction[] {
-            const diagnosticsForCodeFix = this.getDiagnostics(fileName).map(diagnostic => {
-                return {
-                    start: diagnostic.start,
-                    length: diagnostic.length,
-                    code: diagnostic.code
-                };
-            });
-            const dedupedDiagnositcs = ts.deduplicate(diagnosticsForCodeFix, ts.equalOwnProperties);
-
-            let actions: ts.CodeAction[] = undefined;
-
-            for (const diagnostic of dedupedDiagnositcs) {
+            const diagnosticsForCodeFix = this.getDiagnostics(fileName).map(diagnostic => ({
+                start: diagnostic.start,
+                length: diagnostic.length,
+                code: diagnostic.code
+            }));
 
+            return ts.flatMap(ts.deduplicate(diagnosticsForCodeFix, ts.equalOwnProperties), diagnostic => {
                 if (errorCode && errorCode !== diagnostic.code) {
-                    continue;
+                    return;
                 }
 
-                const newActions = this.languageService.getCodeFixesAtPosition(fileName, diagnostic.start, diagnostic.length, [diagnostic.code], this.formatCodeSettings);
-                if (newActions && newActions.length) {
-                    actions = actions ? actions.concat(newActions) : newActions;
-                }
-            }
-            return actions;
+                return this.languageService.getCodeFixesAtPosition(fileName, diagnostic.start, diagnostic.start + diagnostic.length, [diagnostic.code], this.formatCodeSettings);
+            });
         }
 
         private applyCodeActions(actions: ts.CodeAction[], index?: number): void {
             if (index === undefined) {
                 if (!(actions && actions.length === 1)) {
-                    this.raiseError(`Should find exactly one codefix, but ${actions ? actions.length : "none"} found.`);
+                    this.raiseError(`Should find exactly one codefix, but ${actions ? actions.length : "none"} found. ${actions ? actions.map(a => `${Harness.IO.newLine()} "${a.description}"`) : "" }`);
                 }
                 index = 0;
             }
@@ -2373,7 +2386,7 @@ namespace FourSlash {
 
             const codeFixes = this.getCodeFixActions(this.activeFile.fileName, errorCode);
 
-            if (!codeFixes || codeFixes.length === 0) {
+            if (codeFixes.length === 0) {
                 if (expectedTextArray.length !== 0) {
                     this.raiseError("No codefixes returned.");
                 }
@@ -2702,11 +2715,11 @@ namespace FourSlash {
         public verifyCodeFixAvailable(negative: boolean) {
             const codeFix = this.getCodeFixActions(this.activeFile.fileName);
 
-            if (negative && codeFix) {
+            if (negative && codeFix.length) {
                 this.raiseError(`verifyCodeFixAvailable failed - expected no fixes but found one.`);
             }
 
-            if (!(negative || codeFix)) {
+            if (!(negative || codeFix.length)) {
                 this.raiseError(`verifyCodeFixAvailable failed - expected code fixes but none found.`);
             }
         }
@@ -3426,6 +3439,10 @@ namespace FourSlashInterface {
         public markerByName(s: string): FourSlash.Marker {
             return this.state.getMarkerByName(s);
         }
+
+        public symbolsInScope(range: FourSlash.Range): ts.Symbol[] {
+            return this.state.symbolsInScope(range);
+        }
     }
 
     export class GoTo {
@@ -3694,6 +3711,10 @@ namespace FourSlashInterface {
             this.state.verifySymbolAtLocation(startRange, declarationRanges);
         }
 
+        public typeOfSymbolAtLocation(range: FourSlash.Range, symbol: ts.Symbol, expected: string) {
+            this.state.verifyTypeOfSymbolAtLocation(range, symbol, expected);
+        }
+
         public referencesOf(start: FourSlash.Range, references: FourSlash.Range[]) {
             this.state.verifyReferencesOf(start, references);
         }
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index 026ff2de85f..d5218695f09 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -493,7 +493,7 @@ namespace Harness {
         args(): string[];
         getExecutingFilePath(): string;
         exit(exitCode?: number): void;
-        readDirectory(path: string, extension?: string[], exclude?: string[], include?: string[], depth?: number): string[];
+        readDirectory(path: string, extension?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
         tryEnableSourceMapsForHost?(): void;
         getEnvironmentVariable?(name: string): string;
     }
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 82f0b87be94..339138ba30f 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -208,7 +208,7 @@ namespace Harness.LanguageService {
             const script = this.getScriptSnapshot(fileName);
             return script !== undefined;
         }
-        readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[] {
+        readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[] {
             return ts.matchFiles(path, extensions, exclude, include,
                 /*useCaseSensitiveFileNames*/ false,
                 this.getCurrentDirectory(),
diff --git a/src/harness/loggedIO.ts b/src/harness/loggedIO.ts
index d4a1e2e2bd0..f113b9ddb9d 100644
--- a/src/harness/loggedIO.ts
+++ b/src/harness/loggedIO.ts
@@ -64,11 +64,11 @@ interface IOLog {
     }[];
     directoriesRead: {
         path: string,
-        extensions: string[],
-        exclude: string[],
-        include: string[],
+        extensions: ReadonlyArray<string>,
+        exclude: ReadonlyArray<string>,
+        include: ReadonlyArray<string>,
         depth: number,
-        result: string[]
+        result: ReadonlyArray<string>,
     }[];
 }
 
diff --git a/src/harness/unittests/tsserverProjectSystem.ts b/src/harness/unittests/tsserverProjectSystem.ts
index 83e92c2018d..273f7dd1534 100644
--- a/src/harness/unittests/tsserverProjectSystem.ts
+++ b/src/harness/unittests/tsserverProjectSystem.ts
@@ -585,27 +585,25 @@ namespace ts.projectSystem {
             return [];
         }
 
-        readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[] {
+        readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[] {
             return ts.matchFiles(this.toFullPath(path), extensions, exclude, include, this.useCaseSensitiveFileNames, this.getCurrentDirectory(), depth, (dir) => {
-                const result: FileSystemEntries = {
-                    directories: [],
-                    files: []
-                };
+                const directories: string[] = [];
+                const files: string[] = [];
                 const dirEntry = this.fs.get(this.toPath(dir));
                 if (isFolder(dirEntry)) {
                     dirEntry.entries.forEach((entry) => {
                         if (isFolder(entry)) {
-                            result.directories.push(getBaseFileName(entry.fullPath));
+                            directories.push(getBaseFileName(entry.fullPath));
                         }
                         else if (isFile(entry)) {
-                            result.files.push(getBaseFileName(entry.fullPath));
+                            files.push(getBaseFileName(entry.fullPath));
                         }
                         else {
                             Debug.fail("Unknown entry");
                         }
                     });
                 }
-                return result;
+                return { directories, files };
             });
         }
 
diff --git a/src/harness/unittests/typingsInstaller.ts b/src/harness/unittests/typingsInstaller.ts
index b29a619b1b4..287b0fe483d 100644
--- a/src/harness/unittests/typingsInstaller.ts
+++ b/src/harness/unittests/typingsInstaller.ts
@@ -44,6 +44,18 @@ namespace ts.projectSystem {
         });
     }
 
+    function trackingLogger(): { log(message: string): void, finish(): string[] } {
+        const logs: string[] = [];
+        return {
+            log(message) {
+               logs.push(message);
+            },
+            finish() {
+                return logs;
+            }
+        };
+    }
+
     import typingsName = TI.typingsName;
 
     describe("local module", () => {
@@ -1038,7 +1050,12 @@ namespace ts.projectSystem {
             const cache = createMap<string>();
 
             const host = createServerHost([app, jquery, chroma]);
-            const result = JsTyping.discoverTypings(host, [app.path, jquery.path, chroma.path], getDirectoryPath(<Path>app.path), /*safeListPath*/ undefined, cache, { enable: true }, []);
+            const logger = trackingLogger();
+            const result = JsTyping.discoverTypings(host, logger.log, [app.path, jquery.path, chroma.path], getDirectoryPath(<Path>app.path), /*safeListPath*/ undefined, cache, { enable: true }, []);
+            assert.deepEqual(logger.finish(), [
+                'Inferred typings from file names: ["jquery","chroma-js"]',
+                'Result: {"cachedTypingPaths":[],"newTypingNames":["jquery","chroma-js"],"filesToWatch":["/a/b/bower_components","/a/b/node_modules"]}',
+            ]);
             assert.deepEqual(result.newTypingNames, ["jquery", "chroma-js"]);
         });
 
@@ -1051,7 +1068,12 @@ namespace ts.projectSystem {
             const cache = createMap<string>();
 
             for (const name of JsTyping.nodeCoreModuleList) {
-                const result = JsTyping.discoverTypings(host, [f.path], getDirectoryPath(<Path>f.path), /*safeListPath*/ undefined, cache, { enable: true }, [name, "somename"]);
+                const logger = trackingLogger();
+                const result = JsTyping.discoverTypings(host, logger.log, [f.path], getDirectoryPath(<Path>f.path), /*safeListPath*/ undefined, cache, { enable: true }, [name, "somename"]);
+                assert.deepEqual(logger.finish(), [
+                    'Inferred typings from unresolved imports: ["node","somename"]',
+                    'Result: {"cachedTypingPaths":[],"newTypingNames":["node","somename"],"filesToWatch":["/a/b/bower_components","/a/b/node_modules"]}',
+                ]);
                 assert.deepEqual(result.newTypingNames.sort(), ["node", "somename"]);
             }
         });
@@ -1067,7 +1089,12 @@ namespace ts.projectSystem {
             };
             const host = createServerHost([f, node]);
             const cache = createMapFromTemplate<string>({ "node": node.path });
-            const result = JsTyping.discoverTypings(host, [f.path], getDirectoryPath(<Path>f.path), /*safeListPath*/ undefined, cache, { enable: true }, ["fs", "bar"]);
+            const logger = trackingLogger();
+            const result = JsTyping.discoverTypings(host, logger.log, [f.path], getDirectoryPath(<Path>f.path), /*safeListPath*/ undefined, cache, { enable: true }, ["fs", "bar"]);
+            assert.deepEqual(logger.finish(), [
+                'Inferred typings from unresolved imports: ["node","bar"]',
+                'Result: {"cachedTypingPaths":["/a/b/node.d.ts"],"newTypingNames":["bar"],"filesToWatch":["/a/b/bower_components","/a/b/node_modules"]}',
+            ]);
             assert.deepEqual(result.cachedTypingPaths, [node.path]);
             assert.deepEqual(result.newTypingNames, ["bar"]);
         });
@@ -1087,7 +1114,12 @@ namespace ts.projectSystem {
             };
             const host = createServerHost([app, a, b]);
             const cache = createMap<string>();
-            const result = JsTyping.discoverTypings(host, [app.path], getDirectoryPath(<Path>app.path), /*safeListPath*/ undefined, cache, { enable: true }, /*unresolvedImports*/ []);
+            const logger = trackingLogger();
+            const result = JsTyping.discoverTypings(host, logger.log, [app.path], getDirectoryPath(<Path>app.path), /*safeListPath*/ undefined, cache, { enable: true }, /*unresolvedImports*/ []);
+            assert.deepEqual(logger.finish(), [
+                'Searching for typing names in /node_modules; all files: ["/node_modules/a/package.json"]',
+                'Result: {"cachedTypingPaths":[],"newTypingNames":["a"],"filesToWatch":["/bower_components","/node_modules"]}',
+            ]);
             assert.deepEqual(result, {
                 cachedTypingPaths: [],
                 newTypingNames: ["a"], // But not "b"
diff --git a/src/harness/unittests/versionCache.ts b/src/harness/unittests/versionCache.ts
index d6e0a7c2278..10790d41302 100644
--- a/src/harness/unittests/versionCache.ts
+++ b/src/harness/unittests/versionCache.ts
@@ -7,8 +7,7 @@ namespace ts {
     }
 
     function lineColToPosition(lineIndex: server.LineIndex, line: number, col: number) {
-        const lineInfo = lineIndex.lineNumberToInfo(line);
-        return (lineInfo.offset + col - 1);
+        return lineIndex.absolutePositionOfStartOfLine(line) + (col - 1);
     }
 
     function validateEdit(lineIndex: server.LineIndex, sourceText: string, position: number, deleteLength: number, insertString: string): void {
@@ -298,20 +297,17 @@ and grew 1cm per day`;
 
         it("Line/offset from pos", () => {
             for (let i = 0; i < iterationCount; i++) {
-                const lp = lineIndex.charOffsetToLineNumberAndPos(rsa[i]);
+                const lp = lineIndex.positionToLineOffset(rsa[i]);
                 const lac = ts.computeLineAndCharacterOfPosition(lineMap, rsa[i]);
                 assert.equal(lac.line + 1, lp.line, "Line number mismatch " + (lac.line + 1) + " " + lp.line + " " + i);
-                assert.equal(lac.character, (lp.offset), "Charachter offset mismatch " + lac.character + " " + lp.offset + " " + i);
+                assert.equal(lac.character, lp.offset - 1, "Character offset mismatch " + lac.character + " " + (lp.offset - 1) + " " + i);
             }
         });
 
         it("Start pos from line", () => {
             for (let i = 0; i < iterationCount; i++) {
                 for (let j = 0; j < lines.length; j++) {
-                    const lineInfo = lineIndex.lineNumberToInfo(j + 1);
-                    const lineIndexOffset = lineInfo.offset;
-                    const lineMapOffset = lineMap[j];
-                    assert.equal(lineIndexOffset, lineMapOffset);
+                    assert.equal(lineIndex.absolutePositionOfStartOfLine(j + 1), lineMap[j]);
                 }
             }
         });
diff --git a/src/harness/virtualFileSystem.ts b/src/harness/virtualFileSystem.ts
index 3904a27bb1c..da7784770df 100644
--- a/src/harness/virtualFileSystem.ts
+++ b/src/harness/virtualFileSystem.ts
@@ -216,7 +216,7 @@ namespace Utils {
             }
         }
 
-        readDirectory(path: string, extensions: string[], excludes: string[], includes: string[], depth: number) {
+        readDirectory(path: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, depth: number) {
             return ts.matchFiles(path, extensions, excludes, includes, this.useCaseSensitiveFileNames, this.currentDirectory, depth, (path: string) => this.getAccessibleFileSystemEntries(path));
         }
     }
diff --git a/src/server/builder.ts b/src/server/builder.ts
index 7d35247e974..8fd8fbf1e65 100644
--- a/src/server/builder.ts
+++ b/src/server/builder.ts
@@ -277,7 +277,7 @@ namespace ts.server {
 
             const referencedFilePaths = this.project.getReferencedFiles(fileInfo.scriptInfo.path);
             if (referencedFilePaths.length > 0) {
-                return map(referencedFilePaths, f => this.getOrCreateFileInfo(f)).sort(ModuleBuilderFileInfo.compareFileInfos);
+                return map<Path, ModuleBuilderFileInfo>(referencedFilePaths, f => this.getOrCreateFileInfo(f)).sort(ModuleBuilderFileInfo.compareFileInfos);
             }
             return [];
         }
diff --git a/src/server/client.ts b/src/server/client.ts
index f0be887814d..a7e0615dce8 100644
--- a/src/server/client.ts
+++ b/src/server/client.ts
@@ -227,7 +227,7 @@ namespace ts.server {
             }));
         }
 
-        getFormattingEditsForRange(file: string, start: number, end: number, _options: ts.FormatCodeOptions): ts.TextChange[] {
+        getFormattingEditsForRange(file: string, start: number, end: number, _options: FormatCodeOptions): ts.TextChange[] {
             const args: protocol.FormatRequestArgs = this.createFileLocationRequestArgsWithEndLineAndOffset(file, start, end);
 
 
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 6d9be7a6520..1b2793e5f2f 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -45,7 +45,7 @@ namespace ts.server {
          * Any compiler options that might contain paths will be taken out.
          * Enum compiler options will be converted to strings.
          */
-        readonly compilerOptions: ts.CompilerOptions;
+        readonly compilerOptions: CompilerOptions;
         // "extends", "files", "include", or "exclude" will be undefined if an external config is used.
         // Otherwise, we will use "true" if the property is present and "false" if it is missing.
         readonly extends: boolean | undefined;
@@ -201,7 +201,7 @@ namespace ts.server {
      * This helper function processes a list of projects and return the concatenated, sortd and deduplicated output of processing each project.
      */
     export function combineProjectOutput<T>(projects: Project[], action: (project: Project) => T[], comparer?: (a: T, b: T) => number, areEqual?: (a: T, b: T) => boolean) {
-        const result = projects.reduce<T[]>((previous, current) => concatenate(previous, action(current)), []).sort(comparer);
+        const result = flatMap(projects, action).sort(comparer);
         return projects.length > 1 ? deduplicate(result, areEqual) : result;
     }
 
@@ -226,7 +226,7 @@ namespace ts.server {
         getFileName: x => x,
         getScriptKind: _ => undefined,
         hasMixedContent: (fileName, extraFileExtensions) => {
-            const mixedContentExtensions = ts.map(ts.filter(extraFileExtensions, item => item.isMixedContent), item => item.extension);
+            const mixedContentExtensions = map(filter(extraFileExtensions, item => item.isMixedContent), item => item.extension);
             return forEach(mixedContentExtensions, extension => fileExtensionIs(fileName, extension));
         }
     };
@@ -663,7 +663,7 @@ namespace ts.server {
             // If a change was made inside "folder/file", node will trigger the callback twice:
             // one with the fileName being "folder/file", and the other one with "folder".
             // We don't respond to the second one.
-            if (fileName && !ts.isSupportedSourceFileName(fileName, project.getCompilerOptions(), this.hostConfiguration.extraFileExtensions)) {
+            if (fileName && !isSupportedSourceFileName(fileName, project.getCompilerOptions(), this.hostConfiguration.extraFileExtensions)) {
                 return;
             }
 
@@ -1120,7 +1120,7 @@ namespace ts.server {
                 configFileName: configFileName(),
                 projectType: project instanceof server.ExternalProject ? "external" : "configured",
                 languageServiceEnabled: project.languageServiceEnabled,
-                version: ts.version,
+                version,
             };
             this.eventHandler({ eventName: ProjectInfoTelemetryEvent, data });
 
@@ -1130,7 +1130,7 @@ namespace ts.server {
                 }
 
                 const configFilePath = project instanceof server.ConfiguredProject && project.getConfigFilePath();
-                const base = ts.getBaseFileName(configFilePath);
+                const base = getBaseFileName(configFilePath);
                 return base === "tsconfig.json" || base === "jsconfig.json" ? base : "other";
             }
 
diff --git a/src/server/lsHost.ts b/src/server/lsHost.ts
index 133db8cac2c..b54f493be1b 100644
--- a/src/server/lsHost.ts
+++ b/src/server/lsHost.ts
@@ -3,8 +3,6 @@
 /// <reference path="scriptInfo.ts" />
 
 namespace ts.server {
-    type NameResolutionWithFailedLookupLocations = { failedLookupLocations: string[], isInvalidated?: boolean };
-
     export class CachedServerHost implements ServerHost {
         args: string[];
         newLine: string;
@@ -32,7 +30,7 @@ namespace ts.server {
         }
 
         private getFileSystemEntries(rootDir: string) {
-            const path = ts.toPath(rootDir, this.currentDirectory, this.getCanonicalFileName);
+            const path = toPath(rootDir, this.currentDirectory, this.getCanonicalFileName);
             const cachedResult = this.cachedReadDirectoryResult.get(path);
             if (cachedResult) {
                 return cachedResult;
@@ -49,7 +47,7 @@ namespace ts.server {
 
         private canWorkWithCacheForDir(rootDir: string) {
             // Some of the hosts might not be able to handle read directory or getDirectories
-            const path = ts.toPath(rootDir, this.currentDirectory, this.getCanonicalFileName);
+            const path = toPath(rootDir, this.currentDirectory, this.getCanonicalFileName);
             if (this.cachedReadDirectoryResult.get(path)) {
                 return true;
             }
@@ -66,11 +64,11 @@ namespace ts.server {
         }
 
         writeFile(fileName: string, data: string, writeByteOrderMark?: boolean) {
-            const path = ts.toPath(fileName, this.currentDirectory, this.getCanonicalFileName);
+            const path = toPath(fileName, this.currentDirectory, this.getCanonicalFileName);
             const result = this.cachedReadDirectoryResult.get(getDirectoryPath(path));
             const baseFileName = getBaseFileName(toNormalizedPath(fileName));
-            if (result && !some(result.files, file => this.fileNameEqual(file, baseFileName))) {
-                result.files.push(baseFileName);
+            if (result) {
+                result.files = this.updateFileSystemEntry(result.files, baseFileName, /*isValid*/ true);
             }
             return this.host.writeFile(fileName, data, writeByteOrderMark);
         }
@@ -102,7 +100,7 @@ namespace ts.server {
 
         getDirectories(rootDir: string) {
             if (this.canWorkWithCacheForDir(rootDir)) {
-                return this.getFileSystemEntries(rootDir).directories;
+                return this.getFileSystemEntries(rootDir).directories.slice();
             }
             return this.host.getDirectories(rootDir);
         }
@@ -115,14 +113,14 @@ namespace ts.server {
         }
 
         fileExists(fileName: string): boolean {
-            const path = ts.toPath(fileName, this.currentDirectory, this.getCanonicalFileName);
+            const path = toPath(fileName, this.currentDirectory, this.getCanonicalFileName);
             const result = this.cachedReadDirectoryResult.get(getDirectoryPath(path));
             const baseName = getBaseFileName(toNormalizedPath(fileName));
-            return (result && some(result.files, file => this.fileNameEqual(file, baseName))) || this.host.fileExists(fileName);
+            return (result && this.hasEntry(result.files, baseName)) || this.host.fileExists(fileName);
         }
 
         directoryExists(dirPath: string) {
-            const path = ts.toPath(dirPath, this.currentDirectory, this.getCanonicalFileName);
+            const path = toPath(dirPath, this.currentDirectory, this.getCanonicalFileName);
             return this.cachedReadDirectoryResult.has(path) || this.host.directoryExists(dirPath);
         }
 
@@ -134,15 +132,20 @@ namespace ts.server {
             return this.getCanonicalFileName(name1) === this.getCanonicalFileName(name2);
         }
 
-        private updateFileSystemEntry(entries: string[], baseName: string, isValid: boolean) {
-            if (some(entries, entry => this.fileNameEqual(entry, baseName))) {
+        private hasEntry(entries: ReadonlyArray<string>, name: string) {
+            return some(entries, file => this.fileNameEqual(file, name));
+        }
+
+        private updateFileSystemEntry(entries: ReadonlyArray<string>, baseName: string, isValid: boolean) {
+            if (this.hasEntry(entries, baseName)) {
                 if (!isValid) {
-                    filterMutate(entries, entry => !this.fileNameEqual(entry, baseName));
+                    return filter(entries, entry => !this.fileNameEqual(entry, baseName));
                 }
             }
             else if (isValid) {
-                entries.push(baseName);
+                return entries.concat(baseName);
             }
+            return entries;
         }
 
         addOrDeleteFileOrFolder(fileOrFolder: NormalizedPath) {
@@ -159,8 +162,8 @@ namespace ts.server {
                 if (parentResult) {
                     const baseName = getBaseFileName(fileOrFolder);
                     if (parentResult) {
-                        this.updateFileSystemEntry(parentResult.files, baseName, this.host.fileExists(path));
-                        this.updateFileSystemEntry(parentResult.directories, baseName, this.host.directoryExists(path));
+                        parentResult.files = this.updateFileSystemEntry(parentResult.files, baseName, this.host.fileExists(path));
+                        parentResult.directories = this.updateFileSystemEntry(parentResult.directories, baseName, this.host.directoryExists(path));
                     }
                 }
             }
@@ -185,8 +188,9 @@ namespace ts.server {
 
     }
 
-    export class LSHost implements ts.LanguageServiceHost, ModuleResolutionHost {
-        private compilationSettings: ts.CompilerOptions;
+    type NameResolutionWithFailedLookupLocations = { failedLookupLocations: string[], isInvalidated?: boolean };
+    export class LSHost implements LanguageServiceHost, ModuleResolutionHost {
+        private compilationSettings: CompilerOptions;
         private readonly resolvedModuleNames = createMap<Map<ResolvedModuleWithFailedLookupLocations>>();
         private readonly resolvedTypeReferenceDirectives = createMap<Map<ResolvedTypeReferenceDirectiveWithFailedLookupLocations>>();
         /* @internal */
@@ -203,7 +207,7 @@ namespace ts.server {
         constructor(host: ServerHost, private project: Project, private readonly cancellationToken: HostCancellationToken) {
             this.host = host;
             this.cancellationToken = new ThrottledCancellationToken(cancellationToken, project.projectService.throttleWaitMilliseconds);
-            this.getCanonicalFileName = ts.createGetCanonicalFileName(this.host.useCaseSensitiveFileNames);
+            this.getCanonicalFileName = createGetCanonicalFileName(this.host.useCaseSensitiveFileNames);
 
             if (host.trace) {
                 this.trace = s => host.trace(s);
@@ -285,7 +289,7 @@ namespace ts.server {
                     }
                 }
 
-                ts.Debug.assert(resolution !== undefined);
+                Debug.assert(resolution !== undefined);
 
                 resolvedModules.push(getResult(resolution));
             }
@@ -363,7 +367,7 @@ namespace ts.server {
             return combinePaths(nodeModuleBinDir, getDefaultLibFileName(this.compilationSettings));
         }
 
-        getScriptSnapshot(filename: string): ts.IScriptSnapshot {
+        getScriptSnapshot(filename: string): IScriptSnapshot {
             const scriptInfo = this.project.getScriptInfoLSHost(filename);
             if (scriptInfo) {
                 return scriptInfo.getSnapshot();
@@ -411,7 +415,7 @@ namespace ts.server {
             return this.host.directoryExists(path);
         }
 
-        readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[] {
+        readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[] {
             return this.host.readDirectory(path, extensions, exclude, include, depth);
         }
 
@@ -450,7 +454,7 @@ namespace ts.server {
             });
         }
 
-        setCompilationSettings(opt: ts.CompilerOptions) {
+        setCompilationSettings(opt: CompilerOptions) {
             if (changesAffectModuleResolution(this.compilationSettings, opt)) {
                 this.resolvedModuleNames.clear();
                 this.resolvedTypeReferenceDirectives.clear();
diff --git a/src/server/project.ts b/src/server/project.ts
index 5b78f1f6630..9db74efeb6b 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -115,7 +115,7 @@ namespace ts.server {
     export abstract class Project {
         private rootFiles: ScriptInfo[] = [];
         private rootFilesMap: Map<ProjectRoot> = createMap<ProjectRoot>();
-        private program: ts.Program;
+        private program: Program;
         private externalFiles: SortedReadonlyArray<string>;
         private missingFilesMap: Map<FileWatcher> = createMap<FileWatcher>();
 
@@ -191,7 +191,7 @@ namespace ts.server {
             private readonly projectName: string,
             readonly projectKind: ProjectKind,
             readonly projectService: ProjectService,
-            private documentRegistry: ts.DocumentRegistry,
+            private documentRegistry: DocumentRegistry,
             hasExplicitListOfFiles: boolean,
             languageServiceEnabled: boolean,
             private compilerOptions: CompilerOptions,
@@ -199,7 +199,7 @@ namespace ts.server {
             host: ServerHost) {
 
             if (!this.compilerOptions) {
-                this.compilerOptions = ts.getDefaultCompilerOptions();
+                this.compilerOptions = getDefaultCompilerOptions();
                 this.compilerOptions.allowNonTsExtensions = true;
                 this.compilerOptions.allowJs = true;
             }
@@ -213,7 +213,7 @@ namespace ts.server {
             this.lsHost = new LSHost(host, this, this.projectService.cancellationToken);
             this.lsHost.setCompilationSettings(this.compilerOptions);
 
-            this.languageService = ts.createLanguageService(this.lsHost, this.documentRegistry);
+            this.languageService = createLanguageService(this.lsHost, this.documentRegistry);
 
             if (!languageServiceEnabled) {
                 this.disableLanguageService();
@@ -729,7 +729,7 @@ namespace ts.server {
             }
             let strBuilder = "";
             for (const file of this.program.getSourceFiles()) {
-                strBuilder += `${file.fileName}\n`;
+                strBuilder += `\t${file.fileName}\n`;
             }
             return strBuilder;
         }
@@ -914,7 +914,7 @@ namespace ts.server {
         // Used to keep track of what directories are watched for this project
         directoriesWatchedForTsconfig: string[] = [];
 
-        constructor(projectService: ProjectService, documentRegistry: ts.DocumentRegistry, compilerOptions: CompilerOptions) {
+        constructor(projectService: ProjectService, documentRegistry: DocumentRegistry, compilerOptions: CompilerOptions) {
             super(InferredProject.newName(),
                 ProjectKind.Inferred,
                 projectService,
@@ -995,7 +995,7 @@ namespace ts.server {
 
         constructor(configFileName: NormalizedPath,
             projectService: ProjectService,
-            documentRegistry: ts.DocumentRegistry,
+            documentRegistry: DocumentRegistry,
             hasExplicitListOfFiles: boolean,
             compilerOptions: CompilerOptions,
             languageServiceEnabled: boolean,
@@ -1232,7 +1232,7 @@ namespace ts.server {
         }
 
         getEffectiveTypeRoots() {
-            return ts.getEffectiveTypeRoots(this.getCompilerOptions(), this.lsHost.host) || [];
+            return getEffectiveTypeRoots(this.getCompilerOptions(), this.lsHost.host) || [];
         }
     }
 
@@ -1244,7 +1244,7 @@ namespace ts.server {
         private typeAcquisition: TypeAcquisition;
         constructor(public externalProjectName: string,
             projectService: ProjectService,
-            documentRegistry: ts.DocumentRegistry,
+            documentRegistry: DocumentRegistry,
             compilerOptions: CompilerOptions,
             languageServiceEnabled: boolean,
             public compileOnSaveEnabled: boolean,
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index 916d9c51a14..b6756a7d4ff 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -640,7 +640,7 @@ namespace ts.server.protocol {
     }
 
     /**
-     * Location in source code expressed as (one-based) line and character offset.
+     * Location in source code expressed as (one-based) line and (one-based) column offset.
      */
     export interface Location {
         line: number;
diff --git a/src/server/scriptInfo.ts b/src/server/scriptInfo.ts
index 90853124815..3da4cdb14af 100644
--- a/src/server/scriptInfo.ts
+++ b/src/server/scriptInfo.ts
@@ -61,7 +61,7 @@ namespace ts.server {
                 : ScriptSnapshot.fromString(this.getOrLoadText());
         }
 
-        public getLineInfo(line: number) {
+        public getLineInfo(line: number): AbsolutePositionAndLineText {
             return this.switchToScriptVersionCache().getSnapshot().index.lineNumberToInfo(line);
         }
         /**
@@ -72,19 +72,12 @@ namespace ts.server {
                 const lineMap = this.getLineMap();
                 const start = lineMap[line]; // -1 since line is 1-based
                 const end = line + 1 < lineMap.length ? lineMap[line + 1] : this.text.length;
-                return ts.createTextSpanFromBounds(start, end);
+                return createTextSpanFromBounds(start, end);
             }
             const index = this.svc.getSnapshot().index;
-            const lineInfo = index.lineNumberToInfo(line + 1);
-            let len: number;
-            if (lineInfo.leaf) {
-                len = lineInfo.leaf.text.length;
-            }
-            else {
-                const nextLineInfo = index.lineNumberToInfo(line + 2);
-                len = nextLineInfo.offset - lineInfo.offset;
-            }
-            return ts.createTextSpan(lineInfo.offset, len);
+            const { lineText, absolutePosition } = index.lineNumberToInfo(line + 1);
+            const len = lineText !== undefined ? lineText.length : index.absolutePositionOfStartOfLine(line + 2) - absolutePosition;
+            return createTextSpan(absolutePosition, len);
         }
 
         /**
@@ -93,27 +86,19 @@ namespace ts.server {
          */
         lineOffsetToPosition(line: number, offset: number): number {
             if (!this.svc) {
-                return computePositionOfLineAndCharacter(this.getLineMap(), line - 1, offset - 1);
+                return computePositionOfLineAndCharacter(this.getLineMap(), line - 1, offset - 1, this.text);
             }
-            const index = this.svc.getSnapshot().index;
 
-            const lineInfo = index.lineNumberToInfo(line);
             // TODO: assert this offset is actually on the line
-            return (lineInfo.offset + offset - 1);
+            return this.svc.getSnapshot().index.absolutePositionOfStartOfLine(line) + (offset - 1);
         }
 
-        /**
-         * @param line 1-based index
-         * @param offset 1-based index
-         */
-        positionToLineOffset(position: number): ILineInfo {
+        positionToLineOffset(position: number): protocol.Location {
             if (!this.svc) {
                 const { line, character } = computeLineAndCharacterOfPosition(this.getLineMap(), position);
                 return { line: line + 1, offset: character + 1 };
             }
-            const index = this.svc.getSnapshot().index;
-            const lineOffset = index.charOffsetToLineNumberAndPos(position);
-            return { line: lineOffset.line, offset: lineOffset.offset + 1 };
+            return this.svc.getSnapshot().index.positionToLineOffset(position);
         }
 
         private getFileText(tempFileName?: string) {
@@ -162,7 +147,7 @@ namespace ts.server {
          * All projects that include this file
          */
         readonly containingProjects: Project[] = [];
-        private formatCodeSettings: ts.FormatCodeSettings;
+        private formatCodeSettings: FormatCodeSettings;
         readonly path: Path;
 
         private fileWatcher: FileWatcher;
@@ -343,7 +328,7 @@ namespace ts.server {
             }
         }
 
-        getLineInfo(line: number) {
+        getLineInfo(line: number): AbsolutePositionAndLineText {
             return this.textStorage.getLineInfo(line);
         }
 
@@ -373,11 +358,7 @@ namespace ts.server {
             return this.textStorage.lineOffsetToPosition(line, offset);
         }
 
-        /**
-         * @param line 1-based index
-         * @param offset 1-based index
-         */
-        positionToLineOffset(position: number): ILineInfo {
+        positionToLineOffset(position: number): protocol.Location {
             return this.textStorage.positionToLineOffset(position);
         }
 
diff --git a/src/server/scriptVersionCache.ts b/src/server/scriptVersionCache.ts
index 464eea2efab..b6a189904db 100644
--- a/src/server/scriptVersionCache.ts
+++ b/src/server/scriptVersionCache.ts
@@ -8,15 +8,13 @@ namespace ts.server {
     export interface LineCollection {
         charCount(): number;
         lineCount(): number;
-        isLeaf(): boolean;
+        isLeaf(): this is LineLeaf;
         walk(rangeStart: number, rangeLength: number, walkFns: ILineIndexWalker): void;
     }
 
-    export interface ILineInfo {
-        line: number;
-        offset: number;
-        text?: string;
-        leaf?: LineLeaf;
+    export interface AbsolutePositionAndLineText {
+        absolutePosition: number;
+        lineText: string | undefined;
     }
 
     export enum CharRangeSection {
@@ -97,22 +95,20 @@ namespace ts.server {
             }
 
             // path at least length two (root and leaf)
-            let insertionNode = <LineNode>this.startPath[this.startPath.length - 2];
             const leafNode = <LineLeaf>this.startPath[this.startPath.length - 1];
-            const len = lines.length;
 
-            if (len > 0) {
+            if (lines.length > 0) {
                 leafNode.text = lines[0];
 
-                if (len > 1) {
-                    let insertedNodes = <LineCollection[]>new Array(len - 1);
+                if (lines.length > 1) {
+                    let insertedNodes = <LineCollection[]>new Array(lines.length - 1);
                     let startNode = <LineCollection>leafNode;
                     for (let i = 1; i < lines.length; i++) {
                         insertedNodes[i - 1] = new LineLeaf(lines[i]);
                     }
                     let pathIndex = this.startPath.length - 2;
                     while (pathIndex >= 0) {
-                        insertionNode = <LineNode>this.startPath[pathIndex];
+                        const insertionNode = <LineNode>this.startPath[pathIndex];
                         insertedNodes = insertionNode.insertAt(startNode, insertedNodes);
                         pathIndex--;
                         startNode = insertionNode;
@@ -134,6 +130,7 @@ namespace ts.server {
                 }
             }
             else {
+                const insertionNode = <LineNode>this.startPath[this.startPath.length - 2];
                 // no content for leaf node, so delete it
                 insertionNode.remove(leafNode);
                 for (let j = this.startPath.length - 2; j >= 0; j--) {
@@ -251,7 +248,7 @@ namespace ts.server {
         }
 
         getTextChangeRange() {
-            return ts.createTextChangeRange(ts.createTextSpan(this.pos, this.deleteLen),
+            return createTextChangeRange(createTextSpan(this.pos, this.deleteLen),
                 this.insertedText ? this.insertedText.length : 0);
         }
     }
@@ -281,9 +278,9 @@ namespace ts.server {
         // REVIEW: can optimize by coalescing simple edits
         edit(pos: number, deleteLen: number, insertedText?: string) {
             this.changes.push(new TextChange(pos, deleteLen, insertedText));
-            if ((this.changes.length > ScriptVersionCache.changeNumberThreshold) ||
-                (deleteLen > ScriptVersionCache.changeLengthThreshold) ||
-                (insertedText && (insertedText.length > ScriptVersionCache.changeLengthThreshold))) {
+            if (this.changes.length > ScriptVersionCache.changeNumberThreshold ||
+                deleteLen > ScriptVersionCache.changeLengthThreshold ||
+                insertedText && insertedText.length > ScriptVersionCache.changeLengthThreshold) {
                 this.getSnapshot();
             }
         }
@@ -340,21 +337,21 @@ namespace ts.server {
         getTextChangesBetweenVersions(oldVersion: number, newVersion: number) {
             if (oldVersion < newVersion) {
                 if (oldVersion >= this.minVersion) {
-                    const textChangeRanges: ts.TextChangeRange[] = [];
+                    const textChangeRanges: TextChangeRange[] = [];
                     for (let i = oldVersion + 1; i <= newVersion; i++) {
                         const snap = this.versions[this.versionToIndex(i)];
                         for (const textChange of snap.changesSincePreviousVersion) {
                             textChangeRanges.push(textChange.getTextChangeRange());
                         }
                     }
-                    return ts.collapseTextChangeRangesAcrossMultipleVersions(textChangeRanges);
+                    return collapseTextChangeRangesAcrossMultipleVersions(textChangeRanges);
                 }
                 else {
                     return undefined;
                 }
             }
             else {
-                return ts.unchangedTextChangeRange;
+                return unchangedTextChangeRange;
             }
         }
 
@@ -368,7 +365,7 @@ namespace ts.server {
         }
     }
 
-    export class LineIndexSnapshot implements ts.IScriptSnapshot {
+    export class LineIndexSnapshot implements IScriptSnapshot {
         constructor(readonly version: number, readonly cache: ScriptVersionCache, readonly index: LineIndex, readonly changesSincePreviousVersion: ReadonlyArray<TextChange> = emptyArray) {
         }
 
@@ -380,10 +377,10 @@ namespace ts.server {
             return this.index.root.charCount();
         }
 
-        getChangeRange(oldSnapshot: ts.IScriptSnapshot): ts.TextChangeRange {
+        getChangeRange(oldSnapshot: IScriptSnapshot): TextChangeRange {
             if (oldSnapshot instanceof LineIndexSnapshot && this.cache === oldSnapshot.cache) {
                 if (this.version <= oldSnapshot.version) {
-                    return ts.unchangedTextChangeRange;
+                    return unchangedTextChangeRange;
                 }
                 else {
                     return this.cache.getTextChangesBetweenVersions(oldSnapshot.version, this.version);
@@ -397,22 +394,27 @@ namespace ts.server {
         // set this to true to check each edit for accuracy
         checkEdits = false;
 
-        charOffsetToLineNumberAndPos(charOffset: number) {
-            return this.root.charOffsetToLineNumberAndPos(1, charOffset);
+        absolutePositionOfStartOfLine(oneBasedLine: number): number {
+            return this.lineNumberToInfo(oneBasedLine).absolutePosition;
         }
 
-        lineNumberToInfo(lineNumber: number): ILineInfo {
+        positionToLineOffset(position: number): protocol.Location {
+            const { oneBasedLine, zeroBasedColumn } = this.root.charOffsetToLineInfo(1, position);
+            return { line: oneBasedLine, offset: zeroBasedColumn + 1 };
+        }
+
+        private positionToColumnAndLineText(position: number): { zeroBasedColumn: number, lineText: string } {
+            return this.root.charOffsetToLineInfo(1, position);
+        }
+
+        lineNumberToInfo(oneBasedLine: number): AbsolutePositionAndLineText {
             const lineCount = this.root.lineCount();
-            if (lineNumber <= lineCount) {
-                const lineInfo = this.root.lineNumberToInfo(lineNumber, 0);
-                lineInfo.line = lineNumber;
-                return lineInfo;
+            if (oneBasedLine <= lineCount) {
+                const { position, leaf } = this.root.lineNumberToInfo(oneBasedLine, 0);
+                return { absolutePosition: position, lineText: leaf && leaf.text };
             }
             else {
-                return {
-                    line: lineNumber,
-                    offset: this.root.charCount()
-                };
+                return { absolutePosition: this.root.charCount(), lineText: undefined };
             }
         }
 
@@ -468,12 +470,9 @@ namespace ts.server {
             return !walkFns.done;
         }
 
-        edit(pos: number, deleteLength: number, newText?: string) {
-            function editFlat(source: string, s: number, dl: number, nt = "") {
-                return source.substring(0, s) + nt + source.substring(s + dl, source.length);
-            }
+        edit(pos: number, deleteLength: number, newText?: string): LineIndex {
             if (this.root.charCount() === 0) {
-                // TODO: assert deleteLength === 0
+                Debug.assert(deleteLength === 0); // Can't delete from empty document
                 if (newText !== undefined) {
                     this.load(LineIndex.linesFromText(newText).lines);
                     return this;
@@ -482,7 +481,8 @@ namespace ts.server {
             else {
                 let checkText: string;
                 if (this.checkEdits) {
-                    checkText = editFlat(this.getText(0, this.root.charCount()), pos, deleteLength, newText);
+                    const source = this.getText(0, this.root.charCount());
+                    checkText = source.slice(0, pos) + newText + source.slice(pos + deleteLength);
                 }
                 const walker = new EditWalker();
                 let suppressTrailingText = false;
@@ -502,63 +502,44 @@ namespace ts.server {
                 else if (deleteLength > 0) {
                     // check whether last characters deleted are line break
                     const e = pos + deleteLength;
-                    const lineInfo = this.charOffsetToLineNumberAndPos(e);
-                    if ((lineInfo && (lineInfo.offset === 0))) {
+                    const { zeroBasedColumn, lineText } = this.positionToColumnAndLineText(e);
+                    if (zeroBasedColumn === 0) {
                         // move range end just past line that will merge with previous line
-                        deleteLength += lineInfo.text.length;
+                        deleteLength += lineText.length;
                         // store text by appending to end of insertedText
-                        if (newText) {
-                            newText = newText + lineInfo.text;
-                        }
-                        else {
-                            newText = lineInfo.text;
-                        }
+                        newText = newText ? newText + lineText : lineText;
                     }
                 }
-                if (pos < this.root.charCount()) {
-                    this.root.walk(pos, deleteLength, walker);
-                    walker.insertLines(newText, suppressTrailingText);
-                }
+
+                this.root.walk(pos, deleteLength, walker);
+                walker.insertLines(newText, suppressTrailingText);
+
                 if (this.checkEdits) {
-                    const updatedText = this.getText(0, this.root.charCount());
+                    const updatedText = walker.lineIndex.getText(0, walker.lineIndex.getLength());
                     Debug.assert(checkText === updatedText, "buffer edit mismatch");
                 }
+
                 return walker.lineIndex;
             }
         }
 
-        static buildTreeFromBottom(nodes: LineCollection[]): LineNode {
-            const nodeCount = Math.ceil(nodes.length / lineCollectionCapacity);
-            const interiorNodes: LineNode[] = [];
+        private static buildTreeFromBottom(nodes: LineCollection[]): LineNode {
+            if (nodes.length < lineCollectionCapacity) {
+                return new LineNode(nodes);
+            }
+
+            const interiorNodes = new Array<LineNode>(Math.ceil(nodes.length / lineCollectionCapacity));
             let nodeIndex = 0;
-            for (let i = 0; i < nodeCount; i++) {
-                interiorNodes[i] = new LineNode();
-                let charCount = 0;
-                let lineCount = 0;
-                for (let j = 0; j < lineCollectionCapacity; j++) {
-                    if (nodeIndex < nodes.length) {
-                        interiorNodes[i].add(nodes[nodeIndex]);
-                        charCount += nodes[nodeIndex].charCount();
-                        lineCount += nodes[nodeIndex].lineCount();
-                    }
-                    else {
-                        break;
-                    }
-                    nodeIndex++;
-                }
-                interiorNodes[i].totalChars = charCount;
-                interiorNodes[i].totalLines = lineCount;
-            }
-            if (interiorNodes.length === 1) {
-                return interiorNodes[0];
-            }
-            else {
-                return this.buildTreeFromBottom(interiorNodes);
+            for (let i = 0; i < interiorNodes.length; i++) {
+                const end = Math.min(nodeIndex + lineCollectionCapacity, nodes.length);
+                interiorNodes[i] = new LineNode(nodes.slice(nodeIndex, end));
+                nodeIndex = end;
             }
+            return this.buildTreeFromBottom(interiorNodes);
         }
 
         static linesFromText(text: string) {
-            const lineMap = ts.computeLineStarts(text);
+            const lineMap = computeLineStarts(text);
 
             if (lineMap.length === 0) {
                 return { lines: <string[]>[], lineMap };
@@ -583,7 +564,10 @@ namespace ts.server {
     export class LineNode implements LineCollection {
         totalChars = 0;
         totalLines = 0;
-        children: LineCollection[] = [];
+
+        constructor(private readonly children: LineCollection[] = []) {
+            if (children.length) this.updateCounts();
+        }
 
         isLeaf() {
             return false;
@@ -598,7 +582,7 @@ namespace ts.server {
             }
         }
 
-        execWalk(rangeStart: number, rangeLength: number, walkFns: ILineIndexWalker, childIndex: number, nodeType: CharRangeSection) {
+        private execWalk(rangeStart: number, rangeLength: number, walkFns: ILineIndexWalker, childIndex: number, nodeType: CharRangeSection) {
             if (walkFns.pre) {
                 walkFns.pre(rangeStart, rangeLength, this.children[childIndex], this, nodeType);
             }
@@ -614,7 +598,7 @@ namespace ts.server {
             return walkFns.done;
         }
 
-        skipChild(relativeStart: number, relativeLength: number, childIndex: number, walkFns: ILineIndexWalker, nodeType: CharRangeSection) {
+        private skipChild(relativeStart: number, relativeLength: number, childIndex: number, walkFns: ILineIndexWalker, nodeType: CharRangeSection) {
             if (walkFns.pre && (!walkFns.done)) {
                 walkFns.pre(relativeStart, relativeLength, this.children[childIndex], this, nodeType);
                 walkFns.goSubtree = true;
@@ -624,16 +608,14 @@ namespace ts.server {
         walk(rangeStart: number, rangeLength: number, walkFns: ILineIndexWalker) {
             // assume (rangeStart < this.totalChars) && (rangeLength <= this.totalChars)
             let childIndex = 0;
-            let child = this.children[0];
-            let childCharCount = child.charCount();
+            let childCharCount = this.children[childIndex].charCount();
             // find sub-tree containing start
             let adjustedStart = rangeStart;
             while (adjustedStart >= childCharCount) {
                 this.skipChild(adjustedStart, rangeLength, childIndex, walkFns, CharRangeSection.PreStart);
                 adjustedStart -= childCharCount;
                 childIndex++;
-                child = this.children[childIndex];
-                childCharCount = child.charCount();
+                childCharCount = this.children[childIndex].charCount();
             }
             // Case I: both start and end of range in same subtree
             if ((adjustedStart + rangeLength) <= childCharCount) {
@@ -648,7 +630,7 @@ namespace ts.server {
                 }
                 let adjustedLength = rangeLength - (childCharCount - adjustedStart);
                 childIndex++;
-                child = this.children[childIndex];
+                const child = this.children[childIndex];
                 childCharCount = child.charCount();
                 while (adjustedLength > childCharCount) {
                     if (this.execWalk(0, childCharCount, walkFns, childIndex, CharRangeSection.Mid)) {
@@ -656,8 +638,7 @@ namespace ts.server {
                     }
                     adjustedLength -= childCharCount;
                     childIndex++;
-                    child = this.children[childIndex];
-                    childCharCount = child.charCount();
+                    childCharCount = this.children[childIndex].charCount();
                 }
                 if (adjustedLength > 0) {
                     if (this.execWalk(0, adjustedLength, walkFns, childIndex, CharRangeSection.End)) {
@@ -676,90 +657,88 @@ namespace ts.server {
             }
         }
 
-        charOffsetToLineNumberAndPos(lineNumber: number, charOffset: number): ILineInfo {
-            const childInfo = this.childFromCharOffset(lineNumber, charOffset);
+        // Input position is relative to the start of this node.
+        // Output line number is absolute.
+        charOffsetToLineInfo(lineNumberAccumulator: number, relativePosition: number): { oneBasedLine: number, zeroBasedColumn: number, lineText: string | undefined } {
+            const childInfo = this.childFromCharOffset(lineNumberAccumulator, relativePosition);
             if (!childInfo.child) {
                 return {
-                    line: lineNumber,
-                    offset: charOffset,
+                    oneBasedLine: lineNumberAccumulator,
+                    zeroBasedColumn: relativePosition,
+                    lineText: undefined,
                 };
             }
             else if (childInfo.childIndex < this.children.length) {
                 if (childInfo.child.isLeaf()) {
                     return {
-                        line: childInfo.lineNumber,
-                        offset: childInfo.charOffset,
-                        text: (<LineLeaf>(childInfo.child)).text,
-                        leaf: (<LineLeaf>(childInfo.child))
+                        oneBasedLine: childInfo.lineNumberAccumulator,
+                        zeroBasedColumn: childInfo.relativePosition,
+                        lineText: childInfo.child.text,
                     };
                 }
                 else {
                     const lineNode = <LineNode>(childInfo.child);
-                    return lineNode.charOffsetToLineNumberAndPos(childInfo.lineNumber, childInfo.charOffset);
+                    return lineNode.charOffsetToLineInfo(childInfo.lineNumberAccumulator, childInfo.relativePosition);
                 }
             }
             else {
                 const lineInfo = this.lineNumberToInfo(this.lineCount(), 0);
-                return { line: this.lineCount(), offset: lineInfo.leaf.charCount() };
+                return { oneBasedLine: this.lineCount(), zeroBasedColumn: lineInfo.leaf.charCount(), lineText: undefined };
             }
         }
 
-        lineNumberToInfo(lineNumber: number, charOffset: number): ILineInfo {
-            const childInfo = this.childFromLineNumber(lineNumber, charOffset);
+        lineNumberToInfo(relativeOneBasedLine: number, positionAccumulator: number): { position: number, leaf: LineLeaf | undefined } {
+            const childInfo = this.childFromLineNumber(relativeOneBasedLine, positionAccumulator);
             if (!childInfo.child) {
-                return {
-                    line: lineNumber,
-                    offset: charOffset
-                };
+                return { position: positionAccumulator, leaf: undefined };
             }
             else if (childInfo.child.isLeaf()) {
-                return {
-                    line: lineNumber,
-                    offset: childInfo.charOffset,
-                    text: (<LineLeaf>(childInfo.child)).text,
-                    leaf: (<LineLeaf>(childInfo.child))
-                };
+                return { position: childInfo.positionAccumulator, leaf: childInfo.child };
             }
             else {
                 const lineNode = <LineNode>(childInfo.child);
-                return lineNode.lineNumberToInfo(childInfo.relativeLineNumber, childInfo.charOffset);
+                return lineNode.lineNumberToInfo(childInfo.relativeOneBasedLine, childInfo.positionAccumulator);
             }
         }
 
-        childFromLineNumber(lineNumber: number, charOffset: number) {
+        /**
+         * Input line number is relative to the start of this node.
+         * Output line number is relative to the child.
+         * positionAccumulator will be an absolute position once relativeLineNumber reaches 0.
+         */
+        private childFromLineNumber(relativeOneBasedLine: number, positionAccumulator: number): { child: LineCollection, relativeOneBasedLine: number, positionAccumulator: number } {
             let child: LineCollection;
-            let relativeLineNumber = lineNumber;
             let i: number;
-            let len: number;
-            for (i = 0, len = this.children.length; i < len; i++) {
+            for (i = 0; i < this.children.length; i++) {
                 child = this.children[i];
                 const childLineCount = child.lineCount();
-                if (childLineCount >= relativeLineNumber) {
+                if (childLineCount >= relativeOneBasedLine) {
                     break;
                 }
                 else {
-                    relativeLineNumber -= childLineCount;
-                    charOffset += child.charCount();
+                    relativeOneBasedLine -= childLineCount;
+                    positionAccumulator += child.charCount();
                 }
             }
-            return { child, childIndex: i, relativeLineNumber, charOffset };
+            return { child, relativeOneBasedLine, positionAccumulator };
         }
 
-        childFromCharOffset(lineNumber: number, charOffset: number) {
+        private childFromCharOffset(lineNumberAccumulator: number, relativePosition: number
+            ): { child: LineCollection, childIndex: number, relativePosition: number, lineNumberAccumulator: number } {
             let child: LineCollection;
             let i: number;
             let len: number;
             for (i = 0, len = this.children.length; i < len; i++) {
                 child = this.children[i];
-                if (child.charCount() > charOffset) {
+                if (child.charCount() > relativePosition) {
                     break;
                 }
                 else {
-                    charOffset -= child.charCount();
-                    lineNumber += child.lineCount();
+                    relativePosition -= child.charCount();
+                    lineNumberAccumulator += child.lineCount();
                 }
             }
-            return { child, childIndex: i, charOffset, lineNumber };
+            return { child, childIndex: i, relativePosition, lineNumberAccumulator };
         }
 
         private splitAfter(childIndex: number) {
diff --git a/src/server/server.ts b/src/server/server.ts
index 873f7fc613c..f672ec31e1e 100644
--- a/src/server/server.ts
+++ b/src/server/server.ts
@@ -138,7 +138,7 @@ namespace ts.server {
         terminal: false,
     });
 
-    class Logger implements ts.server.Logger {
+    class Logger implements server.Logger {
         private fd = -1;
         private seq = 0;
         private inGroup = false;
@@ -526,12 +526,18 @@ namespace ts.server {
                 if (err) {
                     watchedFile.callback(watchedFile.fileName, FileWatcherEventKind.Changed);
                 }
-                else if (watchedFile.mtime.getTime() !== stats.mtime.getTime()) {
-                    watchedFile.mtime = stats.mtime;
-                    const eventKind = watchedFile.mtime.getTime() === 0
-                        ? FileWatcherEventKind.Deleted
-                        : FileWatcherEventKind.Changed;
-                    watchedFile.callback(watchedFile.fileName, eventKind);
+                else {
+                    const oldTime = watchedFile.mtime.getTime();
+                    const newTime = stats.mtime.getTime();
+                    if (oldTime !== newTime) {
+                        watchedFile.mtime = stats.mtime;
+                        const eventKind = oldTime === 0
+                            ? FileWatcherEventKind.Created
+                            : newTime === 0
+                                ? FileWatcherEventKind.Deleted
+                                : FileWatcherEventKind.Changed;
+                        watchedFile.callback(watchedFile.fileName, eventKind);
+                    }
                 }
             });
         }
diff --git a/src/server/session.ts b/src/server/session.ts
index a51e17cde65..7f1f571c629 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -49,7 +49,7 @@ namespace ts.server {
 
     interface FileStart {
         file: string;
-        start: ILineInfo;
+        start: protocol.Location;
     }
 
     function compareNumber(a: number, b: number) {
@@ -72,28 +72,28 @@ namespace ts.server {
         }
     }
 
-    function formatDiag(fileName: NormalizedPath, project: Project, diag: ts.Diagnostic): protocol.Diagnostic {
+    function formatDiag(fileName: NormalizedPath, project: Project, diag: Diagnostic): protocol.Diagnostic {
         const scriptInfo = project.getScriptInfoForNormalizedPath(fileName);
         return {
             start: scriptInfo.positionToLineOffset(diag.start),
             end: scriptInfo.positionToLineOffset(diag.start + diag.length),
-            text: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
+            text: flattenDiagnosticMessageText(diag.messageText, "\n"),
             code: diag.code,
             category: DiagnosticCategory[diag.category].toLowerCase(),
             source: diag.source
         };
     }
 
-    function convertToILineInfo(lineAndCharacter: LineAndCharacter): ILineInfo {
+    function convertToLocation(lineAndCharacter: LineAndCharacter): protocol.Location {
         return { line: lineAndCharacter.line + 1, offset: lineAndCharacter.character + 1 };
     }
 
-    function formatConfigFileDiag(diag: ts.Diagnostic, includeFileName: true): protocol.DiagnosticWithFileName;
-    function formatConfigFileDiag(diag: ts.Diagnostic, includeFileName: false): protocol.Diagnostic;
-    function formatConfigFileDiag(diag: ts.Diagnostic, includeFileName: boolean): protocol.Diagnostic | protocol.DiagnosticWithFileName {
-        const start = diag.file && convertToILineInfo(getLineAndCharacterOfPosition(diag.file, diag.start));
-        const end = diag.file && convertToILineInfo(getLineAndCharacterOfPosition(diag.file, diag.start + diag.length));
-        const text = ts.flattenDiagnosticMessageText(diag.messageText, "\n");
+    function formatConfigFileDiag(diag: Diagnostic, includeFileName: true): protocol.DiagnosticWithFileName;
+    function formatConfigFileDiag(diag: Diagnostic, includeFileName: false): protocol.Diagnostic;
+    function formatConfigFileDiag(diag: Diagnostic, includeFileName: boolean): protocol.Diagnostic | protocol.DiagnosticWithFileName {
+        const start = diag.file && convertToLocation(getLineAndCharacterOfPosition(diag.file, diag.start));
+        const end = diag.file && convertToLocation(getLineAndCharacterOfPosition(diag.file, diag.start + diag.length));
+        const text = flattenDiagnosticMessageText(diag.messageText, "\n");
         const { code, source } = diag;
         const category = DiagnosticCategory[diag.category].toLowerCase();
         return includeFileName ? { start, end, text, code, category, source, fileName: diag.file && diag.file.fileName } :
@@ -389,8 +389,8 @@ namespace ts.server {
             this.host.write(formatMessage(msg, this.logger, this.byteLength, this.host.newLine));
         }
 
-        public configFileDiagnosticEvent(triggerFile: string, configFile: string, diagnostics: ts.Diagnostic[]) {
-            const bakedDiags = ts.map(diagnostics, diagnostic => formatConfigFileDiag(diagnostic, /*includeFileName*/ true));
+        public configFileDiagnosticEvent(triggerFile: string, configFile: string, diagnostics: Diagnostic[]) {
+            const bakedDiags = map(diagnostics, diagnostic => formatConfigFileDiag(diagnostic, /*includeFileName*/ true));
             const ev: protocol.ConfigFileDiagnosticEvent = {
                 seq: 0,
                 type: "event",
@@ -547,8 +547,8 @@ namespace ts.server {
                 length: d.length,
                 category: DiagnosticCategory[d.category].toLowerCase(),
                 code: d.code,
-                startLocation: d.file && convertToILineInfo(getLineAndCharacterOfPosition(d.file, d.start)),
-                endLocation: d.file && convertToILineInfo(getLineAndCharacterOfPosition(d.file, d.start + d.length))
+                startLocation: d.file && convertToLocation(getLineAndCharacterOfPosition(d.file, d.start)),
+                endLocation: d.file && convertToLocation(getLineAndCharacterOfPosition(d.file, d.start + d.length))
             });
         }
 
@@ -607,7 +607,7 @@ namespace ts.server {
                     return {
                         file: def.fileName,
                         start: defScriptInfo.positionToLineOffset(def.textSpan.start),
-                        end: defScriptInfo.positionToLineOffset(ts.textSpanEnd(def.textSpan))
+                        end: defScriptInfo.positionToLineOffset(textSpanEnd(def.textSpan))
                     };
                 });
             }
@@ -631,7 +631,7 @@ namespace ts.server {
                 return {
                     file: def.fileName,
                     start: defScriptInfo.positionToLineOffset(def.textSpan.start),
-                    end: defScriptInfo.positionToLineOffset(ts.textSpanEnd(def.textSpan))
+                    end: defScriptInfo.positionToLineOffset(textSpanEnd(def.textSpan))
                 };
             });
         }
@@ -649,7 +649,7 @@ namespace ts.server {
                     return {
                         file: fileName,
                         start: scriptInfo.positionToLineOffset(textSpan.start),
-                        end: scriptInfo.positionToLineOffset(ts.textSpanEnd(textSpan))
+                        end: scriptInfo.positionToLineOffset(textSpanEnd(textSpan))
                     };
                 });
             }
@@ -673,7 +673,7 @@ namespace ts.server {
                 const { fileName, isWriteAccess, textSpan, isInString } = occurrence;
                 const scriptInfo = project.getScriptInfo(fileName);
                 const start = scriptInfo.positionToLineOffset(textSpan.start);
-                const end = scriptInfo.positionToLineOffset(ts.textSpanEnd(textSpan));
+                const end = scriptInfo.positionToLineOffset(textSpanEnd(textSpan));
                 const result: protocol.OccurrencesResponseItem = {
                     start,
                     end,
@@ -723,7 +723,7 @@ namespace ts.server {
                 return documentHighlights;
             }
 
-            function convertToDocumentHighlightsItem(documentHighlights: ts.DocumentHighlights): ts.server.protocol.DocumentHighlightsItem {
+            function convertToDocumentHighlightsItem(documentHighlights: DocumentHighlights): protocol.DocumentHighlightsItem {
                 const { fileName, highlightSpans } = documentHighlights;
 
                 const scriptInfo = project.getScriptInfo(fileName);
@@ -732,10 +732,10 @@ namespace ts.server {
                     highlightSpans: highlightSpans.map(convertHighlightSpan)
                 };
 
-                function convertHighlightSpan(highlightSpan: ts.HighlightSpan): ts.server.protocol.HighlightSpan {
+                function convertHighlightSpan(highlightSpan: HighlightSpan): protocol.HighlightSpan {
                     const { textSpan, kind } = highlightSpan;
                     const start = scriptInfo.positionToLineOffset(textSpan.start);
-                    const end = scriptInfo.positionToLineOffset(ts.textSpanEnd(textSpan));
+                    const end = scriptInfo.positionToLineOffset(textSpanEnd(textSpan));
                     return { start, end, kind };
                 }
             }
@@ -778,7 +778,7 @@ namespace ts.server {
                 const scriptInfo = this.projectService.getScriptInfo(args.file);
                 projects = scriptInfo.containingProjects;
             }
-            // ts.filter handles case when 'projects' is undefined
+            // filter handles case when 'projects' is undefined
             projects = filter(projects, p => p.languageServiceEnabled);
             if (!projects || !projects.length) {
                 return Errors.ThrowNoProject();
@@ -831,28 +831,29 @@ namespace ts.server {
                             return <protocol.FileSpan>{
                                 file: location.fileName,
                                 start: locationScriptInfo.positionToLineOffset(location.textSpan.start),
-                                end: locationScriptInfo.positionToLineOffset(ts.textSpanEnd(location.textSpan)),
+                                end: locationScriptInfo.positionToLineOffset(textSpanEnd(location.textSpan)),
                             };
                         });
                     },
                     compareRenameLocation,
                     (a, b) => a.file === b.file && a.start.line === b.start.line && a.start.offset === b.start.offset
                 );
-                const locs = fileSpans.reduce<protocol.SpanGroup[]>((accum, cur) => {
+
+                const locs: protocol.SpanGroup[] = [];
+                for (const cur of fileSpans) {
                     let curFileAccum: protocol.SpanGroup;
-                    if (accum.length > 0) {
-                        curFileAccum = accum[accum.length - 1];
+                    if (locs.length > 0) {
+                        curFileAccum = locs[locs.length - 1];
                         if (curFileAccum.file !== cur.file) {
                             curFileAccum = undefined;
                         }
                     }
                     if (!curFileAccum) {
                         curFileAccum = { file: cur.file, locs: [] };
-                        accum.push(curFileAccum);
+                        locs.push(curFileAccum);
                     }
                     curFileAccum.locs.push({ start: cur.start, end: cur.end });
-                    return accum;
-                }, []);
+                }
 
                 return { info: renameInfo, locs };
             }
@@ -912,10 +913,10 @@ namespace ts.server {
                     return undefined;
                 }
 
-                const displayString = ts.displayPartsToString(nameInfo.displayParts);
+                const displayString = displayPartsToString(nameInfo.displayParts);
                 const nameSpan = nameInfo.textSpan;
                 const nameColStart = scriptInfo.positionToLineOffset(nameSpan.start).offset;
-                const nameText = scriptInfo.getSnapshot().getText(nameSpan.start, ts.textSpanEnd(nameSpan));
+                const nameText = scriptInfo.getSnapshot().getText(nameSpan.start, textSpanEnd(nameSpan));
                 const refs = combineProjectOutput<protocol.ReferencesResponseItem>(
                     projects,
                     (project: Project) => {
@@ -928,12 +929,12 @@ namespace ts.server {
                             const refScriptInfo = project.getScriptInfo(ref.fileName);
                             const start = refScriptInfo.positionToLineOffset(ref.textSpan.start);
                             const refLineSpan = refScriptInfo.lineToTextSpan(start.line - 1);
-                            const lineText = refScriptInfo.getSnapshot().getText(refLineSpan.start, ts.textSpanEnd(refLineSpan)).replace(/\r|\n/g, "");
+                            const lineText = refScriptInfo.getSnapshot().getText(refLineSpan.start, textSpanEnd(refLineSpan)).replace(/\r|\n/g, "");
                             return {
                                 file: ref.fileName,
                                 start,
                                 lineText,
-                                end: refScriptInfo.positionToLineOffset(ts.textSpanEnd(ref.textSpan)),
+                                end: refScriptInfo.positionToLineOffset(textSpanEnd(ref.textSpan)),
                                 isWriteAccess: ref.isWriteAccess,
                                 isDefinition: ref.isDefinition
                             };
@@ -1056,14 +1057,14 @@ namespace ts.server {
             }
 
             if (simplifiedResult) {
-                const displayString = ts.displayPartsToString(quickInfo.displayParts);
-                const docString = ts.displayPartsToString(quickInfo.documentation);
+                const displayString = displayPartsToString(quickInfo.displayParts);
+                const docString = displayPartsToString(quickInfo.documentation);
 
                 return {
                     kind: quickInfo.kind,
                     kindModifiers: quickInfo.kindModifiers,
                     start: scriptInfo.positionToLineOffset(quickInfo.textSpan.start),
-                    end: scriptInfo.positionToLineOffset(ts.textSpanEnd(quickInfo.textSpan)),
+                    end: scriptInfo.positionToLineOffset(textSpanEnd(quickInfo.textSpan)),
                     displayString,
                     documentation: docString,
                     tags: quickInfo.tags || []
@@ -1123,32 +1124,29 @@ namespace ts.server {
             // only to the previous line.  If all this is true, then
             // add edits necessary to properly indent the current line.
             if ((args.key === "\n") && ((!edits) || (edits.length === 0) || allEditsBeforePos(edits, position))) {
-                const lineInfo = scriptInfo.getLineInfo(args.line);
-                if (lineInfo && (lineInfo.leaf) && (lineInfo.leaf.text)) {
-                    const lineText = lineInfo.leaf.text;
-                    if (lineText.search("\\S") < 0) {
-                        const preferredIndent = project.getLanguageService(/*ensureSynchronized*/ false).getIndentationAtPosition(file, position, formatOptions);
-                        let hasIndent = 0;
-                        let i: number, len: number;
-                        for (i = 0, len = lineText.length; i < len; i++) {
-                            if (lineText.charAt(i) === " ") {
-                                hasIndent++;
-                            }
-                            else if (lineText.charAt(i) === "\t") {
-                                hasIndent += formatOptions.tabSize;
-                            }
-                            else {
-                                break;
-                            }
+                const { lineText, absolutePosition } = scriptInfo.getLineInfo(args.line);
+                if (lineText && lineText.search("\\S") < 0) {
+                    const preferredIndent = project.getLanguageService(/*ensureSynchronized*/ false).getIndentationAtPosition(file, position, formatOptions);
+                    let hasIndent = 0;
+                    let i: number, len: number;
+                    for (i = 0, len = lineText.length; i < len; i++) {
+                        if (lineText.charAt(i) === " ") {
+                            hasIndent++;
                         }
-                        // i points to the first non whitespace character
-                        if (preferredIndent !== hasIndent) {
-                            const firstNoWhiteSpacePosition = lineInfo.offset + i;
-                            edits.push({
-                                span: ts.createTextSpanFromBounds(lineInfo.offset, firstNoWhiteSpacePosition),
-                                newText: formatting.getIndentationString(preferredIndent, formatOptions)
-                            });
+                        else if (lineText.charAt(i) === "\t") {
+                            hasIndent += formatOptions.tabSize;
                         }
+                        else {
+                            break;
+                        }
+                    }
+                    // i points to the first non whitespace character
+                    if (preferredIndent !== hasIndent) {
+                        const firstNoWhiteSpacePosition = absolutePosition + i;
+                        edits.push({
+                            span: createTextSpanFromBounds(absolutePosition, firstNoWhiteSpacePosition),
+                            newText: formatting.getIndentationString(preferredIndent, formatOptions)
+                        });
                     }
                 }
             }
@@ -1160,7 +1158,7 @@ namespace ts.server {
             return edits.map((edit) => {
                 return {
                     start: scriptInfo.positionToLineOffset(edit.span.start),
-                    end: scriptInfo.positionToLineOffset(ts.textSpanEnd(edit.span)),
+                    end: scriptInfo.positionToLineOffset(textSpanEnd(edit.span)),
                     newText: edit.newText ? edit.newText : ""
                 };
             });
@@ -1178,15 +1176,13 @@ namespace ts.server {
                 return undefined;
             }
             if (simplifiedResult) {
-                return completions.entries.reduce((result: protocol.CompletionEntry[], entry: ts.CompletionEntry) => {
+                return mapDefined(completions.entries, entry => {
                     if (completions.isMemberCompletion || (entry.name.toLowerCase().indexOf(prefix.toLowerCase()) === 0)) {
                         const { name, kind, kindModifiers, sortText, replacementSpan } = entry;
-                        const convertedSpan: protocol.TextSpan =
-                            replacementSpan ? this.decorateSpan(replacementSpan, scriptInfo) : undefined;
-                        result.push({ name, kind, kindModifiers, sortText, replacementSpan: convertedSpan });
+                        const convertedSpan = replacementSpan ? this.decorateSpan(replacementSpan, scriptInfo) : undefined;
+                        return { name, kind, kindModifiers, sortText, replacementSpan: convertedSpan };
                     }
-                    return result;
-                }, []).sort((a, b) => ts.compareStrings(a.name, b.name));
+                }).sort((a, b) => compareStrings(a.name, b.name));
             }
             else {
                 return completions;
@@ -1198,13 +1194,8 @@ namespace ts.server {
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const position = this.getPosition(args, scriptInfo);
 
-            return args.entryNames.reduce((accum: protocol.CompletionEntryDetails[], entryName: string) => {
-                const details = project.getLanguageService().getCompletionEntryDetails(file, position, entryName);
-                if (details) {
-                    accum.push(details);
-                }
-                return accum;
-            }, []);
+            return mapDefined(args.entryNames, entryName =>
+                project.getLanguageService().getCompletionEntryDetails(file, position, entryName));
         }
 
         private getCompileOnSaveAffectedFileList(args: protocol.FileRequestArgs): protocol.CompileOnSaveAffectedFileListSingleProject[] {
@@ -1269,14 +1260,11 @@ namespace ts.server {
         }
 
         private getDiagnostics(next: NextStep, delay: number, fileNames: string[]): void {
-            const checkList = fileNames.reduce((accum: PendingErrorCheck[], uncheckedFileName: string) => {
+            const checkList = mapDefined(fileNames, uncheckedFileName => {
                 const fileName = toNormalizedPath(uncheckedFileName);
                 const project = this.projectService.getDefaultProjectForFile(fileName, /*refreshInferredProjects*/ true);
-                if (project) {
-                    accum.push({ fileName, project });
-                }
-                return accum;
-            }, []);
+                return project && { fileName, project };
+            });
 
             if (checkList.length > 0) {
                 this.updateErrorCheck(next, checkList, this.changeSeq, (n) => n === this.changeSeq, delay);
@@ -1321,11 +1309,11 @@ namespace ts.server {
             if (!fileName) {
                 return;
             }
-            const file = ts.normalizePath(fileName);
+            const file = normalizePath(fileName);
             this.projectService.closeClientFile(file);
         }
 
-        private decorateNavigationBarItems(items: ts.NavigationBarItem[], scriptInfo: ScriptInfo): protocol.NavigationBarItem[] {
+        private decorateNavigationBarItems(items: NavigationBarItem[], scriptInfo: ScriptInfo): protocol.NavigationBarItem[] {
             return map(items, item => ({
                 text: item.text,
                 kind: item.kind,
@@ -1346,7 +1334,7 @@ namespace ts.server {
                     : items;
         }
 
-        private decorateNavigationTree(tree: ts.NavigationTree, scriptInfo: ScriptInfo): protocol.NavigationTree {
+        private decorateNavigationTree(tree: NavigationTree, scriptInfo: ScriptInfo): protocol.NavigationTree {
             return {
                 text: tree.text,
                 kind: tree.kind,
@@ -1359,7 +1347,7 @@ namespace ts.server {
         private decorateSpan(span: TextSpan, scriptInfo: ScriptInfo): protocol.TextSpan {
             return {
                 start: scriptInfo.positionToLineOffset(span.start),
-                end: scriptInfo.positionToLineOffset(ts.textSpanEnd(span))
+                end: scriptInfo.positionToLineOffset(textSpanEnd(span))
             };
         }
 
@@ -1389,7 +1377,7 @@ namespace ts.server {
                         return navItems.map((navItem) => {
                             const scriptInfo = project.getScriptInfo(navItem.fileName);
                             const start = scriptInfo.positionToLineOffset(navItem.textSpan.start);
-                            const end = scriptInfo.positionToLineOffset(ts.textSpanEnd(navItem.textSpan));
+                            const end = scriptInfo.positionToLineOffset(textSpanEnd(navItem.textSpan));
                             const bakedItem: protocol.NavtoItem = {
                                 name: navItem.name,
                                 kind: navItem.kind,
@@ -1454,7 +1442,7 @@ namespace ts.server {
         }
 
         private getSupportedCodeFixes(): string[] {
-            return ts.getSupportedCodeFixes();
+            return getSupportedCodeFixes();
         }
 
         private isLocation(locationOrSpan: protocol.FileLocationOrRangeRequestArgs): locationOrSpan is protocol.FileLocationRequestArgs {
@@ -1485,7 +1473,7 @@ namespace ts.server {
             return project.getLanguageService().getApplicableRefactors(file, position || textRange);
         }
 
-        private getEditsForRefactor(args: protocol.GetEditsForRefactorRequestArgs, simplifiedResult: boolean): ts.RefactorEditInfo | protocol.RefactorEditInfo {
+        private getEditsForRefactor(args: protocol.GetEditsForRefactorRequestArgs, simplifiedResult: boolean): RefactorEditInfo | protocol.RefactorEditInfo {
             const { file, project } = this.getFileAndProjectWithoutRefreshingInferredProjects(args);
             const scriptInfo = project.getScriptInfoForNormalizedPath(file);
             const { position, textRange } = this.extractPositionAndRange(args, scriptInfo);
@@ -1506,7 +1494,7 @@ namespace ts.server {
 
             if (simplifiedResult) {
                 const file = result.renameFilename;
-                let location: ILineInfo | undefined  = undefined;
+                let location: protocol.Location | undefined;
                 if (file !== undefined && result.renameLocation !== undefined) {
                     const renameScriptInfo = project.getScriptInfoForNormalizedPath(toNormalizedPath(file));
                     location = renameScriptInfo.positionToLineOffset(result.renameLocation);
@@ -1654,7 +1642,7 @@ namespace ts.server {
 
         getCanonicalFileName(fileName: string) {
             const name = this.host.useCaseSensitiveFileNames ? fileName : fileName.toLowerCase();
-            return ts.normalizePath(name);
+            return normalizePath(name);
         }
 
         exit() {
diff --git a/src/server/types.ts b/src/server/types.ts
index 81e1f09639f..72d6a7c6cfc 100644
--- a/src/server/types.ts
+++ b/src/server/types.ts
@@ -31,9 +31,9 @@ declare namespace ts.server {
 
     export interface DiscoverTypings extends TypingInstallerRequest {
         readonly fileNames: string[];
-        readonly projectRootPath: ts.Path;
-        readonly compilerOptions: ts.CompilerOptions;
-        readonly typeAcquisition: ts.TypeAcquisition;
+        readonly projectRootPath: Path;
+        readonly compilerOptions: CompilerOptions;
+        readonly typeAcquisition: TypeAcquisition;
         readonly unresolvedImports: SortedReadonlyArray<string>;
         readonly cachePath?: string;
         readonly kind: "discover";
@@ -63,8 +63,8 @@ declare namespace ts.server {
     }
 
     export interface SetTypings extends ProjectResponse {
-        readonly typeAcquisition: ts.TypeAcquisition;
-        readonly compilerOptions: ts.CompilerOptions;
+        readonly typeAcquisition: TypeAcquisition;
+        readonly compilerOptions: CompilerOptions;
         readonly typings: string[];
         readonly unresolvedImports: SortedReadonlyArray<string>;
         readonly kind: ActionSet;
diff --git a/src/server/typingsInstaller/typingsInstaller.ts b/src/server/typingsInstaller/typingsInstaller.ts
index 71ad0171bf3..730257d2ccc 100644
--- a/src/server/typingsInstaller/typingsInstaller.ts
+++ b/src/server/typingsInstaller/typingsInstaller.ts
@@ -145,6 +145,7 @@ namespace ts.server.typingsInstaller {
 
             const discoverTypingsResult = JsTyping.discoverTypings(
                 this.installTypingHost,
+                this.log.isEnabled() ? this.log.writeLine : undefined,
                 req.fileNames,
                 req.projectRootPath,
                 this.safeListPath,
diff --git a/src/server/utilities.ts b/src/server/utilities.ts
index 416b148fc0d..3949e7b58dd 100644
--- a/src/server/utilities.ts
+++ b/src/server/utilities.ts
@@ -77,7 +77,7 @@ namespace ts.server {
             tabSize: 4,
             newLineCharacter: host.newLine || "\n",
             convertTabsToSpaces: true,
-            indentStyle: ts.IndentStyle.Smart,
+            indentStyle: IndentStyle.Smart,
             insertSpaceAfterConstructor: false,
             insertSpaceAfterCommaDelimiter: true,
             insertSpaceAfterSemicolonInForStatements: true,
@@ -196,7 +196,7 @@ namespace ts.server {
     }
 
     export function enumerateInsertsAndDeletes<T>(a: SortedReadonlyArray<T>, b: SortedReadonlyArray<T>, inserted: (item: T) => void, deleted: (item: T) => void, compare?: (a: T, b: T) => Comparison) {
-        compare = compare || ts.compareValues;
+        compare = compare || compareValues;
         let aIndex = 0;
         let bIndex = 0;
         const aLen = a.length;
diff --git a/src/services/codefixes/fixUnusedIdentifier.ts b/src/services/codefixes/fixUnusedIdentifier.ts
index cbe2ba5b1c5..18491aaba26 100644
--- a/src/services/codefixes/fixUnusedIdentifier.ts
+++ b/src/services/codefixes/fixUnusedIdentifier.ts
@@ -25,15 +25,15 @@ namespace ts.codefix {
                     return [deleteNode(token.parent)];
 
                 default:
-                    return [deleteDefault()];
+                    return deleteDefault();
             }
 
-            function deleteDefault() {
+            function deleteDefault(): CodeAction[] | undefined {
                 if (isDeclarationName(token)) {
-                    return deleteNode(token.parent);
+                    return [deleteNode(token.parent)];
                 }
                 else if (isLiteralComputedPropertyDeclarationName(token)) {
-                    return deleteNode(token.parent.parent);
+                    return [deleteNode(token.parent.parent)];
                 }
                 else {
                     return undefined;
@@ -87,20 +87,16 @@ namespace ts.codefix {
                     case SyntaxKind.ImportSpecifier:
                         const namedImports = <NamedImports>parent.parent;
                         if (namedImports.elements.length === 1) {
-                            // Only 1 import and it is unused. So the entire declaration should be removed.
-                            const importSpec = getAncestor(identifier, SyntaxKind.ImportDeclaration);
-                            return [deleteNode(importSpec)];
+                            return deleteNamedImportBinding(namedImports);
                         }
                         else {
                             // delete import specifier
                             return [deleteNodeInList(parent)];
                         }
 
-                    // handle case where "import d, * as ns from './file'"
-                    // or "'import {a, b as ns} from './file'"
                     case SyntaxKind.ImportClause: // this covers both 'import |d|' and 'import |d,| *'
                         const importClause = <ImportClause>parent;
-                        if (!importClause.namedBindings) { // |import d from './file'| or |import * as ns from './file'|
+                        if (!importClause.namedBindings) { // |import d from './file'|
                             const importDecl = getAncestor(importClause, SyntaxKind.ImportDeclaration);
                             return [deleteNode(importDecl)];
                         }
@@ -118,22 +114,30 @@ namespace ts.codefix {
                         }
 
                     case SyntaxKind.NamespaceImport:
-                        const namespaceImport = <NamespaceImport>parent;
-                        if (namespaceImport.name === identifier && !(<ImportClause>namespaceImport.parent).name) {
-                            const importDecl = getAncestor(namespaceImport, SyntaxKind.ImportDeclaration);
-                            return [deleteNode(importDecl)];
-                        }
-                        else {
-                            const previousToken = getTokenAtPosition(sourceFile, namespaceImport.pos - 1, /*includeJsDocComment*/ false);
-                            if (previousToken && previousToken.kind === SyntaxKind.CommaToken) {
-                                const startPosition = textChanges.getAdjustedStartPosition(sourceFile, previousToken, {}, textChanges.Position.FullStart);
-                                return [deleteRange({ pos: startPosition, end: namespaceImport.end })];
-                            }
-                            return [deleteRange(namespaceImport)];
-                        }
+                        return deleteNamedImportBinding(<NamespaceImport>parent);
 
                     default:
-                        return [deleteDefault()];
+                        return deleteDefault();
+                }
+            }
+
+            function deleteNamedImportBinding(namedBindings: NamedImportBindings): CodeAction[] | undefined {
+                if ((<ImportClause>namedBindings.parent).name) {
+                    // Delete named imports while preserving the default import
+                    // import d|, * as ns| from './file'
+                    // import d|, { a }| from './file'
+                    const previousToken = getTokenAtPosition(sourceFile, namedBindings.pos - 1, /*includeJsDocComment*/ false);
+                    if (previousToken && previousToken.kind === SyntaxKind.CommaToken) {
+                        return [deleteRange({ pos: previousToken.getStart(), end: namedBindings.end })];
+                    }
+                    return undefined;
+                }
+                else {
+                    // Delete the entire import declaration
+                    // |import * as ns from './file'|
+                    // |import { a } from './file'|
+                    const importDecl = getAncestor(namedBindings, SyntaxKind.ImportDeclaration);
+                    return [deleteNode(importDecl)];
                 }
             }
 
diff --git a/src/services/codefixes/importFixes.ts b/src/services/codefixes/importFixes.ts
index 0fa0b72162b..d30997aa076 100644
--- a/src/services/codefixes/importFixes.ts
+++ b/src/services/codefixes/importFixes.ts
@@ -222,10 +222,7 @@ namespace ts.codefix {
         }
 
         function getUniqueSymbolId(symbol: Symbol) {
-            if (symbol.flags & SymbolFlags.Alias) {
-                return getSymbolId(checker.getAliasedSymbol(symbol));
-            }
-            return getSymbolId(symbol);
+            return getSymbolId(skipAlias(symbol, checker));
         }
 
         function checkSymbolHasMeaning(symbol: Symbol, meaning: SemanticMeaning) {
diff --git a/src/services/completions.ts b/src/services/completions.ts
index 94e8779d1d7..12796a9b791 100644
--- a/src/services/completions.ts
+++ b/src/services/completions.ts
@@ -57,11 +57,11 @@ namespace ts.Completions {
 
         if (isSourceFileJavaScript(sourceFile)) {
             const uniqueNames = getCompletionEntriesFromSymbols(symbols, entries, location, /*performCharacterChecks*/ true, typeChecker, compilerOptions.target, log);
-            addRange(entries, getJavaScriptCompletionEntries(sourceFile, location.pos, uniqueNames, compilerOptions.target));
+            getJavaScriptCompletionEntries(sourceFile, location.pos, uniqueNames, compilerOptions.target, entries);
         }
         else {
             if ((!symbols || symbols.length === 0) && keywordFilters === KeywordCompletionFilters.None) {
-                    return undefined;
+                return undefined;
             }
 
             getCompletionEntriesFromSymbols(symbols, entries, location, /*performCharacterChecks*/ true, typeChecker, compilerOptions.target, log);
@@ -79,40 +79,41 @@ namespace ts.Completions {
         return { isGlobalCompletion, isMemberCompletion, isNewIdentifierLocation, entries };
     }
 
-    function getJavaScriptCompletionEntries(sourceFile: SourceFile, position: number, uniqueNames: Map<true>, target: ScriptTarget): CompletionEntry[] {
-        const entries: CompletionEntry[] = [];
-
-        const nameTable = getNameTable(sourceFile);
-        nameTable.forEach((pos, name) => {
+    function getJavaScriptCompletionEntries(
+        sourceFile: SourceFile,
+        position: number,
+        uniqueNames: Map<true>,
+        target: ScriptTarget,
+        entries: Push<CompletionEntry>): void {
+        getNameTable(sourceFile).forEach((pos, name) => {
             // Skip identifiers produced only from the current location
             if (pos === position) {
                 return;
             }
             const realName = unescapeLeadingUnderscores(name);
 
-            if (!uniqueNames.get(realName)) {
-                uniqueNames.set(realName, true);
-                const displayName = getCompletionEntryDisplayName(realName, target, /*performCharacterChecks*/ true);
-                if (displayName) {
-                    const entry = {
-                        name: displayName,
-                        kind: ScriptElementKind.warning,
-                        kindModifiers: "",
-                        sortText: "1"
-                    };
-                    entries.push(entry);
-                }
+            if (uniqueNames.has(realName)) {
+                return;
+            }
+
+            uniqueNames.set(realName, true);
+            const displayName = getCompletionEntryDisplayName(realName, target, /*performCharacterChecks*/ true);
+            if (displayName) {
+                entries.push({
+                    name: displayName,
+                    kind: ScriptElementKind.warning,
+                    kindModifiers: "",
+                    sortText: "1"
+                });
             }
         });
-
-        return entries;
     }
 
     function createCompletionEntry(symbol: Symbol, location: Node, performCharacterChecks: boolean, typeChecker: TypeChecker, target: ScriptTarget): CompletionEntry {
         // Try to get a valid display name for this symbol, if we could not find one, then ignore it.
         // We would like to only show things that can be added after a dot, so for instance numeric properties can
         // not be accessed with a dot (a.1 <- invalid)
-        const displayName = getCompletionEntryDisplayNameForSymbol(typeChecker, symbol, target, performCharacterChecks, location);
+        const displayName = getCompletionEntryDisplayNameForSymbol(symbol, target, performCharacterChecks);
         if (!displayName) {
             return undefined;
         }
@@ -141,7 +142,7 @@ namespace ts.Completions {
                 const entry = createCompletionEntry(symbol, location, performCharacterChecks, typeChecker, target);
                 if (entry) {
                     const id = entry.name;
-                    if (!uniqueNames.get(id)) {
+                    if (!uniqueNames.has(id)) {
                         entries.push(entry);
                         uniqueNames.set(id, true);
                     }
@@ -307,7 +308,7 @@ namespace ts.Completions {
             // We don't need to perform character checks here because we're only comparing the
             // name against 'entryName' (which is known to be good), not building a new
             // completion entry.
-            const symbol = forEach(symbols, s => getCompletionEntryDisplayNameForSymbol(typeChecker, s, compilerOptions.target, /*performCharacterChecks*/ false, location) === entryName ? s : undefined);
+            const symbol = forEach(symbols, s => getCompletionEntryDisplayNameForSymbol(s, compilerOptions.target, /*performCharacterChecks*/ false) === entryName ? s : undefined);
 
             if (symbol) {
                 const { displayParts, documentation, symbolKind, tags } = SymbolDisplay.getSymbolDisplayPartsDocumentationAndSymbolKind(typeChecker, symbol, sourceFile, location, location, SemanticMeaning.All);
@@ -341,20 +342,14 @@ namespace ts.Completions {
         return undefined;
     }
 
-    export function getCompletionEntrySymbol(typeChecker: TypeChecker, log: (message: string) => void, compilerOptions: CompilerOptions, sourceFile: SourceFile, position: number, entryName: string): Symbol {
+    export function getCompletionEntrySymbol(typeChecker: TypeChecker, log: (message: string) => void, compilerOptions: CompilerOptions, sourceFile: SourceFile, position: number, entryName: string): Symbol | undefined {
         // Compute all the completion symbols again.
         const completionData = getCompletionData(typeChecker, log, sourceFile, position);
-        if (completionData) {
-            const { symbols, location } = completionData;
-
-            // Find the symbol with the matching entry name.
-            // We don't need to perform character checks here because we're only comparing the
-            // name against 'entryName' (which is known to be good), not building a new
-            // completion entry.
-            return forEach(symbols, s => getCompletionEntryDisplayNameForSymbol(typeChecker, s, compilerOptions.target, /*performCharacterChecks*/ false, location) === entryName ? s : undefined);
-        }
-
-        return undefined;
+        // Find the symbol with the matching entry name.
+        // We don't need to perform character checks here because we're only comparing the
+        // name against 'entryName' (which is known to be good), not building a new
+        // completion entry.
+        return completionData && forEach(completionData.symbols, s => getCompletionEntryDisplayNameForSymbol(s, compilerOptions.target, /*performCharacterChecks*/ false) === entryName ? s : undefined);
     }
 
     interface CompletionData {
@@ -369,7 +364,7 @@ namespace ts.Completions {
     }
     type Request = { kind: "JsDocTagName" } | { kind: "JsDocTag" } | { kind: "JsDocParameterName", tag: JSDocParameterTag };
 
-    function getCompletionData(typeChecker: TypeChecker, log: (message: string) => void, sourceFile: SourceFile, position: number): CompletionData {
+    function getCompletionData(typeChecker: TypeChecker, log: (message: string) => void, sourceFile: SourceFile, position: number): CompletionData | undefined {
         const isJavaScriptFile = isSourceFileJavaScript(sourceFile);
 
         let request: Request | undefined;
@@ -601,58 +596,60 @@ namespace ts.Completions {
             isNewIdentifierLocation = false;
 
             // Since this is qualified name check its a type node location
-            const isTypeLocation = isPartOfTypeNode(node.parent) || insideJsDocTagTypeExpression;
+            const isTypeLocation = insideJsDocTagTypeExpression || isPartOfTypeNode(node.parent);
             const isRhsOfImportDeclaration = isInRightSideOfInternalImportEqualsDeclaration(node);
-            if (node.kind === SyntaxKind.Identifier || node.kind === SyntaxKind.QualifiedName || node.kind === SyntaxKind.PropertyAccessExpression) {
+            if (isEntityName(node)) {
                 let symbol = typeChecker.getSymbolAtLocation(node);
+                if (symbol) {
+                    symbol = skipAlias(symbol, typeChecker);
 
-                // This is an alias, follow what it aliases
-                if (symbol && symbol.flags & SymbolFlags.Alias) {
-                    symbol = typeChecker.getAliasedSymbol(symbol);
-                }
-
-                if (symbol && symbol.flags & SymbolFlags.HasExports) {
-                    // Extract module or enum members
-                    const exportedSymbols = typeChecker.getExportsOfModule(symbol);
-                    const isValidValueAccess = (symbol: Symbol) => typeChecker.isValidPropertyAccess(<PropertyAccessExpression>(node.parent), symbol.getUnescapedName());
-                    const isValidTypeAccess = (symbol: Symbol) => symbolCanbeReferencedAtTypeLocation(symbol);
-                    const isValidAccess = isRhsOfImportDeclaration ?
-                        // Any kind is allowed when dotting off namespace in internal import equals declaration
-                        (symbol: Symbol) => isValidTypeAccess(symbol) || isValidValueAccess(symbol) :
-                        isTypeLocation ? isValidTypeAccess : isValidValueAccess;
-                    forEach(exportedSymbols, symbol => {
-                        if (isValidAccess(symbol)) {
-                            symbols.push(symbol);
+                    if (symbol.flags & (SymbolFlags.Module | SymbolFlags.Enum)) {
+                        // Extract module or enum members
+                        const exportedSymbols = typeChecker.getExportsOfModule(symbol);
+                        const isValidValueAccess = (symbol: Symbol) => typeChecker.isValidPropertyAccess(<PropertyAccessExpression>(node.parent), symbol.getUnescapedName());
+                        const isValidTypeAccess = (symbol: Symbol) => symbolCanBeReferencedAtTypeLocation(symbol);
+                        const isValidAccess = isRhsOfImportDeclaration ?
+                            // Any kind is allowed when dotting off namespace in internal import equals declaration
+                            (symbol: Symbol) => isValidTypeAccess(symbol) || isValidValueAccess(symbol) :
+                            isTypeLocation ? isValidTypeAccess : isValidValueAccess;
+                        for (const symbol of exportedSymbols) {
+                            if (isValidAccess(symbol)) {
+                                symbols.push(symbol);
+                            }
                         }
-                    });
+
+                        // If the module is merged with a value, we must get the type of the class and add its propertes (for inherited static methods).
+                        if (!isTypeLocation && symbol.declarations.some(d => d.kind !== SyntaxKind.SourceFile && d.kind !== SyntaxKind.ModuleDeclaration && d.kind !== SyntaxKind.EnumDeclaration)) {
+                            addTypeProperties(typeChecker.getTypeOfSymbolAtLocation(symbol, node));
+                        }
+
+                        return;
+                    }
                 }
             }
 
             if (!isTypeLocation) {
-                const type = typeChecker.getTypeAtLocation(node);
-                addTypeProperties(type);
+                addTypeProperties(typeChecker.getTypeAtLocation(node));
             }
         }
 
         function addTypeProperties(type: Type) {
-            if (type) {
-                // Filter private properties
-                for (const symbol of type.getApparentProperties()) {
-                    if (typeChecker.isValidPropertyAccess(<PropertyAccessExpression>(node.parent), symbol.getUnescapedName())) {
-                        symbols.push(symbol);
-                    }
+            // Filter private properties
+            for (const symbol of type.getApparentProperties()) {
+                if (typeChecker.isValidPropertyAccess(<PropertyAccessExpression>(node.parent), symbol.getUnescapedName())) {
+                    symbols.push(symbol);
                 }
+            }
 
-                if (isJavaScriptFile && type.flags & TypeFlags.Union) {
-                    // In javascript files, for union types, we don't just get the members that
-                    // the individual types have in common, we also include all the members that
-                    // each individual type has.  This is because we're going to add all identifiers
-                    // anyways.  So we might as well elevate the members that were at least part
-                    // of the individual types to a higher status since we know what they are.
-                    const unionType = <UnionType>type;
-                    for (const elementType of unionType.types) {
-                        addTypeProperties(elementType);
-                    }
+            if (isJavaScriptFile && type.flags & TypeFlags.Union) {
+                // In javascript files, for union types, we don't just get the members that
+                // the individual types have in common, we also include all the members that
+                // each individual type has.  This is because we're going to add all identifiers
+                // anyways.  So we might as well elevate the members that were at least part
+                // of the individual types to a higher status since we know what they are.
+                const unionType = <UnionType>type;
+                for (const elementType of unionType.types) {
+                    addTypeProperties(elementType);
                 }
             }
         }
@@ -777,12 +774,12 @@ namespace ts.Completions {
                         (!isContextTokenValueLocation(contextToken) &&
                             (isPartOfTypeNode(location) || isContextTokenTypeLocation(contextToken)))) {
                         // Its a type, but you can reach it by namespace.type as well
-                        return symbolCanbeReferencedAtTypeLocation(symbol);
+                        return symbolCanBeReferencedAtTypeLocation(symbol);
                     }
                 }
 
                 // expressions are value space (which includes the value namespaces)
-                return !!(symbol.flags & SymbolFlags.Value);
+                return !!(getCombinedLocalAndExportSymbolFlags(symbol) & SymbolFlags.Value);
             });
         }
 
@@ -812,21 +809,21 @@ namespace ts.Completions {
             }
         }
 
-        function symbolCanbeReferencedAtTypeLocation(symbol: Symbol): boolean {
+        function symbolCanBeReferencedAtTypeLocation(symbol: Symbol): boolean {
+            symbol = symbol.exportSymbol || symbol;
+
             // This is an alias, follow what it aliases
-            if (symbol && symbol.flags & SymbolFlags.Alias) {
-                symbol = typeChecker.getAliasedSymbol(symbol);
-            }
+            symbol = skipAlias(symbol, typeChecker);
 
             if (symbol.flags & SymbolFlags.Type) {
                 return true;
             }
 
-            if (symbol.flags & (SymbolFlags.ValueModule | SymbolFlags.NamespaceModule)) {
+            if (symbol.flags & SymbolFlags.Module) {
                 const exportedSymbols = typeChecker.getExportsOfModule(symbol);
                 // If the exported symbols contains type,
                 // symbol can be referenced at locations where type is allowed
-                return forEach(exportedSymbols, symbolCanbeReferencedAtTypeLocation);
+                return forEach(exportedSymbols, symbolCanBeReferencedAtTypeLocation);
             }
         }
 
@@ -1598,22 +1595,23 @@ namespace ts.Completions {
     /**
      * Get the name to be display in completion from a given symbol.
      *
-     * @return undefined if the name is of external module otherwise a name with striped of any quote
+     * @return undefined if the name is of external module
      */
-    function getCompletionEntryDisplayNameForSymbol(typeChecker: TypeChecker, symbol: Symbol, target: ScriptTarget, performCharacterChecks: boolean, location: Node): string {
-        const displayName: string = getDeclaredName(typeChecker, symbol, location);
+    function getCompletionEntryDisplayNameForSymbol(symbol: Symbol, target: ScriptTarget, performCharacterChecks: boolean): string | undefined {
+        const name = symbol.getUnescapedName();
+        if (!name) return undefined;
 
-        if (displayName) {
-            const firstCharCode = displayName.charCodeAt(0);
-            // First check of the displayName is not external module; if it is an external module, it is not valid entry
-            if ((symbol.flags & SymbolFlags.Namespace) && (firstCharCode === CharacterCodes.singleQuote || firstCharCode === CharacterCodes.doubleQuote)) {
+        // First check of the displayName is not external module; if it is an external module, it is not valid entry
+        if (symbol.flags & SymbolFlags.Namespace) {
+            const firstCharCode = name.charCodeAt(0);
+            if (firstCharCode === CharacterCodes.singleQuote || firstCharCode === CharacterCodes.doubleQuote) {
                 // If the symbol is external module, don't show it in the completion list
                 // (i.e declare module "http" { const x; } | // <= request completion here, "http" should not be there)
                 return undefined;
             }
         }
 
-        return getCompletionEntryDisplayName(displayName, target, performCharacterChecks);
+        return getCompletionEntryDisplayName(name, target, performCharacterChecks);
     }
 
     /**
@@ -1621,24 +1619,12 @@ namespace ts.Completions {
      * and checking whether the name is valid identifier name.
      */
     function getCompletionEntryDisplayName(name: string, target: ScriptTarget, performCharacterChecks: boolean): string {
-        if (!name) {
-            return undefined;
-        }
-
-        name = stripQuotes(name);
-
-        if (!name) {
-            return undefined;
-        }
-
         // If the user entered name for the symbol was quoted, removing the quotes is not enough, as the name could be an
         // invalid identifier name. We need to check if whatever was inside the quotes is actually a valid identifier name.
         // e.g "b a" is valid quoted name but when we strip off the quotes, it is invalid.
         // We, thus, need to check if whatever was inside the quotes is actually a valid identifier name.
-        if (performCharacterChecks) {
-            if (!isIdentifierText(name, target)) {
-                return undefined;
-            }
+        if (performCharacterChecks && !isIdentifierText(name, target)) {
+            return undefined;
         }
 
         return name;
diff --git a/src/services/importTracker.ts b/src/services/importTracker.ts
index ecd52baa7dd..f17406b16f1 100644
--- a/src/services/importTracker.ts
+++ b/src/services/importTracker.ts
@@ -440,7 +440,7 @@ namespace ts.FindAllReferences {
 
         function getExport(): ExportedSymbol | ImportedSymbol | undefined {
             const parent = node.parent!;
-            if (symbol.flags & SymbolFlags.Export) {
+            if (symbol.exportSymbol) {
                 if (parent.kind === SyntaxKind.PropertyAccessExpression) {
                     // When accessing an export of a JS module, there's no alias. The symbol will still be flagged as an export even though we're at the use.
                     // So check that we are at the declaration.
@@ -449,9 +449,7 @@ namespace ts.FindAllReferences {
                         : undefined;
                 }
                 else {
-                    const { exportSymbol } = symbol;
-                    Debug.assert(!!exportSymbol);
-                    return exportInfo(exportSymbol, getExportKindForDeclaration(parent));
+                    return exportInfo(symbol.exportSymbol, getExportKindForDeclaration(parent));
                 }
             }
             else {
diff --git a/src/services/jsTyping.ts b/src/services/jsTyping.ts
index ff90d5bdb34..f31276b6498 100644
--- a/src/services/jsTyping.ts
+++ b/src/services/jsTyping.ts
@@ -12,7 +12,7 @@ namespace ts.JsTyping {
         directoryExists: (path: string) => boolean;
         fileExists: (fileName: string) => boolean;
         readFile: (path: string, encoding?: string) => string;
-        readDirectory: (rootDir: string, extensions: string[], excludes: string[], includes: string[], depth?: number) => string[];
+        readDirectory: (rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, depth?: number) => string[];
     }
 
     interface PackageJson {
@@ -51,6 +51,7 @@ namespace ts.JsTyping {
      */
     export function discoverTypings(
         host: TypingResolutionHost,
+        log: ((message: string) => void) | undefined,
         fileNames: string[],
         projectRootPath: Path,
         safeListPath: Path,
@@ -107,8 +108,9 @@ namespace ts.JsTyping {
 
         // add typings for unresolved imports
         if (unresolvedImports) {
-            for (const moduleId of unresolvedImports) {
-                const typingName = nodeCoreModules.has(moduleId) ? "node" : moduleId;
+            const x = unresolvedImports.map(moduleId => nodeCoreModules.has(moduleId) ? "node" : moduleId);
+            if (x.length && log) log(`Inferred typings from unresolved imports: ${JSON.stringify(x)}`);
+            for (const typingName of x) {
                 if (!inferredTypings.has(typingName)) {
                     inferredTypings.set(typingName, undefined);
                 }
@@ -136,7 +138,9 @@ namespace ts.JsTyping {
                 newTypingNames.push(typing);
             }
         });
-        return { cachedTypingPaths, newTypingNames, filesToWatch };
+        const result = { cachedTypingPaths, newTypingNames, filesToWatch };
+        if (log) log(`Result: ${JSON.stringify(result)}`);
+        return result;
 
         function addInferredTyping(typingName: string) {
             if (!inferredTypings.has(typingName)) {
@@ -153,6 +157,7 @@ namespace ts.JsTyping {
             }
 
             filesToWatch.push(jsonPath);
+            if (log) log(`Searching for typing names in '${jsonPath}' dependencies`);
             const jsonConfig: PackageJson = readConfigFile(jsonPath, (path: string) => host.readFile(path)).config;
             addInferredTypingsFromKeys(jsonConfig.dependencies);
             addInferredTypingsFromKeys(jsonConfig.devDependencies);
@@ -175,19 +180,23 @@ namespace ts.JsTyping {
          * @param fileNames are the names for source files in the project
          */
         function getTypingNamesFromSourceFileNames(fileNames: string[]) {
-            for (const j of fileNames) {
-                if (!hasJavaScriptFileExtension(j)) continue;
+            const fromFileNames = mapDefined(fileNames, j => {
+                if (!hasJavaScriptFileExtension(j)) return undefined;
 
                 const inferredTypingName = removeFileExtension(getBaseFileName(j.toLowerCase()));
                 const cleanedTypingName = inferredTypingName.replace(/((?:\.|-)min(?=\.|$))|((?:-|\.)\d+)/g, "");
-                const safe = safeList.get(cleanedTypingName);
-                if (safe !== undefined) {
+                return safeList.get(cleanedTypingName);
+            });
+            if (fromFileNames.length) {
+                if (log) log(`Inferred typings from file names: ${JSON.stringify(fromFileNames)}`);
+                for (const safe of fromFileNames) {
                     addInferredTyping(safe);
                 }
             }
 
             const hasJsxFile = some(fileNames, f => fileExtensionIs(f, Extension.Jsx));
             if (hasJsxFile) {
+                if (log) log(`Inferred 'react' typings due to presence of '.jsx' extension`);
                 addInferredTyping("react");
             }
         }
@@ -206,6 +215,7 @@ namespace ts.JsTyping {
 
             // depth of 2, so we access `node_modules/foo` but not `node_modules/foo/bar`
             const fileNames = host.readDirectory(packagesFolderPath, [".json"], /*excludes*/ undefined, /*includes*/ undefined, /*depth*/ 2);
+            if (log) log(`Searching for typing names in ${packagesFolderPath}; all files: ${JSON.stringify(fileNames)}`);
             for (const fileName of fileNames) {
                 const normalizedFileName = normalizePath(fileName);
                 const baseFileName = getBaseFileName(normalizedFileName);
diff --git a/src/services/pathCompletions.ts b/src/services/pathCompletions.ts
index bc94fb7dbc5..9b645280a1e 100644
--- a/src/services/pathCompletions.ts
+++ b/src/services/pathCompletions.ts
@@ -40,19 +40,14 @@ namespace ts.Completions.PathCompletions {
         rootDirs = map(rootDirs, rootDirectory => normalizePath(isRootedDiskPath(rootDirectory) ? rootDirectory : combinePaths(basePath, rootDirectory)));
 
         // Determine the path to the directory containing the script relative to the root directory it is contained within
-        let relativeDirectory: string;
-        for (const rootDirectory of rootDirs) {
-            if (containsPath(rootDirectory, scriptPath, basePath, ignoreCase)) {
-                relativeDirectory = scriptPath.substr(rootDirectory.length);
-                break;
-            }
-        }
+        const relativeDirectory = forEach(rootDirs, rootDirectory =>
+            containsPath(rootDirectory, scriptPath, basePath, ignoreCase) ? scriptPath.substr(rootDirectory.length) : undefined);
 
         // Now find a path for each potential directory that is to be merged with the one containing the script
         return deduplicate(map(rootDirs, rootDirectory => combinePaths(rootDirectory, relativeDirectory)));
     }
 
-    function getCompletionEntriesForDirectoryFragmentWithRootDirs(rootDirs: string[], fragment: string, scriptPath: string, extensions: string[], includeExtensions: boolean, span: TextSpan, compilerOptions: CompilerOptions, host: LanguageServiceHost, exclude?: string): CompletionEntry[] {
+    function getCompletionEntriesForDirectoryFragmentWithRootDirs(rootDirs: string[], fragment: string, scriptPath: string, extensions: ReadonlyArray<string>, includeExtensions: boolean, span: TextSpan, compilerOptions: CompilerOptions, host: LanguageServiceHost, exclude?: string): CompletionEntry[] {
         const basePath = compilerOptions.project || host.getCurrentDirectory();
         const ignoreCase = !(host.useCaseSensitiveFileNames && host.useCaseSensitiveFileNames());
         const baseDirectories = getBaseDirectoriesFromRootDirs(rootDirs, basePath, scriptPath, ignoreCase);
@@ -69,7 +64,7 @@ namespace ts.Completions.PathCompletions {
     /**
      * Given a path ending at a directory, gets the completions for the path, and filters for those entries containing the basename.
      */
-    function getCompletionEntriesForDirectoryFragment(fragment: string, scriptPath: string, extensions: string[], includeExtensions: boolean, span: TextSpan, host: LanguageServiceHost, exclude?: string, result: CompletionEntry[] = []): CompletionEntry[] {
+    function getCompletionEntriesForDirectoryFragment(fragment: string, scriptPath: string, extensions: ReadonlyArray<string>, includeExtensions: boolean, span: TextSpan, host: LanguageServiceHost, exclude?: string, result: CompletionEntry[] = []): CompletionEntry[] {
         if (fragment === undefined) {
             fragment = "";
         }
@@ -190,7 +185,7 @@ namespace ts.Completions.PathCompletions {
         return result;
     }
 
-    function getModulesForPathsPattern(fragment: string, baseUrl: string, pattern: string, fileExtensions: string[], host: LanguageServiceHost): string[] {
+    function getModulesForPathsPattern(fragment: string, baseUrl: string, pattern: string, fileExtensions: ReadonlyArray<string>, host: LanguageServiceHost): string[] {
         if (host.readDirectory) {
             const parsed = hasZeroOrOneAsteriskCharacter(pattern) ? tryParsePattern(pattern) : undefined;
             if (parsed) {
@@ -505,7 +500,7 @@ namespace ts.Completions.PathCompletions {
         return tryIOAndConsumeErrors(host, host.getDirectories, directoryName);
     }
 
-    function tryReadDirectory(host: LanguageServiceHost, path: string, extensions?: string[], exclude?: string[], include?: string[]): string[] {
+    function tryReadDirectory(host: LanguageServiceHost, path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>): string[] {
         return tryIOAndConsumeErrors(host, host.readDirectory, path, extensions, exclude, include);
     }
 
diff --git a/src/services/refactorProvider.ts b/src/services/refactorProvider.ts
index 3c02ddf671c..432df8c53d0 100644
--- a/src/services/refactorProvider.ts
+++ b/src/services/refactorProvider.ts
@@ -33,22 +33,9 @@ namespace ts {
             refactors.set(refactor.name, refactor);
         }
 
-        export function getApplicableRefactors(context: RefactorContext): ApplicableRefactorInfo[] | undefined {
-            let results: ApplicableRefactorInfo[];
-            const refactorList: Refactor[] = [];
-            refactors.forEach(refactor => {
-                refactorList.push(refactor);
-            });
-            for (const refactor of refactorList) {
-                if (context.cancellationToken && context.cancellationToken.isCancellationRequested()) {
-                    return results;
-                }
-                const infos = refactor.getAvailableActions(context);
-                if (infos && infos.length) {
-                    (results || (results = [])).push(...infos);
-                }
-            }
-            return results;
+        export function getApplicableRefactors(context: RefactorContext): ApplicableRefactorInfo[] {
+            return flatMapIter(refactors.values(), refactor =>
+                context.cancellationToken && context.cancellationToken.isCancellationRequested() ? [] : refactor.getAvailableActions(context));
         }
 
         export function getEditsForRefactor(context: RefactorContext, refactorName: string, actionName: string): RefactorEditInfo | undefined {
diff --git a/src/services/services.ts b/src/services/services.ts
index 28a29352b80..7cb70ed5a07 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1760,21 +1760,14 @@ namespace ts {
         function getCodeFixesAtPosition(fileName: string, start: number, end: number, errorCodes: number[], formatOptions: FormatCodeSettings): CodeAction[] {
             synchronizeHostData();
             const sourceFile = getValidSourceFile(fileName);
-            const span = { start, length: end - start };
+            const span = createTextSpanFromBounds(start, end);
             const newLineCharacter = getNewLineOrDefaultFromHost(host);
+            const rulesProvider = getRuleProvider(formatOptions);
 
-            let allFixes: CodeAction[] = [];
-
-            forEach(deduplicate(errorCodes), errorCode => {
+            return flatMap(deduplicate(errorCodes), errorCode => {
                 cancellationToken.throwIfCancellationRequested();
-                const rulesProvider = getRuleProvider(formatOptions);
-                const fixes = codefix.getFixes({ errorCode, sourceFile, span, program, newLineCharacter, host, cancellationToken, rulesProvider });
-                if (fixes) {
-                    allFixes = allFixes.concat(fixes);
-                }
+                return codefix.getFixes({ errorCode, sourceFile, span, program, newLineCharacter, host, cancellationToken, rulesProvider });
             });
-
-            return allFixes;
         }
 
         function getDocCommentTemplateAtPosition(fileName: string, position: number): TextInsertion {
@@ -2055,42 +2048,33 @@ namespace ts {
     }
 
     function initializeNameTable(sourceFile: SourceFile): void {
-        const nameTable = createUnderscoreEscapedMap<number>();
-
-        walk(sourceFile);
-        sourceFile.nameTable = nameTable;
-
-        function walk(node: Node) {
-            switch (node.kind) {
-                case SyntaxKind.Identifier:
-                    setNameTable((<Identifier>node).text, node);
-                    break;
-                case SyntaxKind.StringLiteral:
-                case SyntaxKind.NumericLiteral:
-                    // We want to store any numbers/strings if they were a name that could be
-                    // related to a declaration.  So, if we have 'import x = require("something")'
-                    // then we want 'something' to be in the name table.  Similarly, if we have
-                    // "a['propname']" then we want to store "propname" in the name table.
-                    if (isDeclarationName(node) ||
-                        node.parent.kind === SyntaxKind.ExternalModuleReference ||
-                        isArgumentOfElementAccessExpression(node) ||
-                        isLiteralComputedPropertyDeclarationName(node)) {
-                        setNameTable(getEscapedTextOfIdentifierOrLiteral((<LiteralExpression>node)), node);
-                    }
-                    break;
-                default:
-                    forEachChild(node, walk);
-                    if (node.jsDoc) {
-                        for (const jsDoc of node.jsDoc) {
-                            forEachChild(jsDoc, walk);
-                        }
-                    }
+        const nameTable = sourceFile.nameTable = createUnderscoreEscapedMap<number>();
+        sourceFile.forEachChild(function walk(node) {
+            if ((isIdentifier(node) || isStringOrNumericLiteral(node) && literalIsName(node)) && node.text) {
+                const text = getEscapedTextOfIdentifierOrLiteral(node);
+                nameTable.set(text, nameTable.get(text) === undefined ? node.pos : -1);
             }
-        }
 
-        function setNameTable(text: __String, node: ts.Node): void {
-            nameTable.set(text, nameTable.get(text) === undefined ? node.pos : -1);
-        }
+            forEachChild(node, walk);
+            if (node.jsDoc) {
+                for (const jsDoc of node.jsDoc) {
+                    forEachChild(jsDoc, walk);
+                }
+            }
+        });
+    }
+
+    /**
+     * We want to store any numbers/strings if they were a name that could be
+     * related to a declaration.  So, if we have 'import x = require("something")'
+     * then we want 'something' to be in the name table.  Similarly, if we have
+     * "a['propname']" then we want to store "propname" in the name table.
+     */
+    function literalIsName(node: ts.StringLiteral | ts.NumericLiteral): boolean {
+        return isDeclarationName(node) ||
+            node.parent.kind === SyntaxKind.ExternalModuleReference ||
+            isArgumentOfElementAccessExpression(node) ||
+            isLiteralComputedPropertyDeclarationName(node);
     }
 
     function isObjectLiteralElement(node: Node): node is ObjectLiteralElement  {
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 0a0b2c82c5b..2ee1853f18f 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -444,7 +444,7 @@ namespace ts {
             return this.shimHost.getDefaultLibFileName(JSON.stringify(options));
         }
 
-        public readDirectory(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[] {
+        public readDirectory(path: string, extensions?: ReadonlyArray<string>, exclude?: string[], include?: string[], depth?: number): string[] {
             const pattern = getFileMatcherPatterns(path, exclude, include,
                 this.shimHost.useCaseSensitiveFileNames(), this.shimHost.getCurrentDirectory());
             return JSON.parse(this.shimHost.readDirectory(
@@ -483,7 +483,7 @@ namespace ts {
             }
         }
 
-        public readDirectory(rootDir: string, extensions: string[], exclude: string[], include: string[], depth?: number): string[] {
+        public readDirectory(rootDir: string, extensions: ReadonlyArray<string>, exclude: ReadonlyArray<string>, include: ReadonlyArray<string>, depth?: number): string[] {
             const pattern = getFileMatcherPatterns(rootDir, exclude, include,
                 this.shimHost.useCaseSensitiveFileNames(), this.shimHost.getCurrentDirectory());
             return JSON.parse(this.shimHost.readDirectory(
@@ -1116,6 +1116,7 @@ namespace ts {
                 const info = <DiscoverTypingsInfo>JSON.parse(discoverTypingsJson);
                 return ts.JsTyping.discoverTypings(
                     this.host,
+                    msg => this.logger.log(msg),
                     info.fileNames,
                     toPath(info.projectRootPath, info.projectRootPath, getCanonicalFileName),
                     toPath(info.safeListPath, info.safeListPath, getCanonicalFileName),
diff --git a/src/services/symbolDisplay.ts b/src/services/symbolDisplay.ts
index b599839cfd3..0cb3916c9c5 100644
--- a/src/services/symbolDisplay.ts
+++ b/src/services/symbolDisplay.ts
@@ -2,7 +2,7 @@
 namespace ts.SymbolDisplay {
     // TODO(drosen): use contextual SemanticMeaning.
     export function getSymbolKind(typeChecker: TypeChecker, symbol: Symbol, location: Node): ScriptElementKind {
-        const { flags } = symbol;
+        const flags = getCombinedLocalAndExportSymbolFlags(symbol);
 
         if (flags & SymbolFlags.Class) {
             return getDeclarationOfKind(symbol, SyntaxKind.ClassExpression) ?
@@ -34,7 +34,7 @@ namespace ts.SymbolDisplay {
         if (location.kind === SyntaxKind.ThisKeyword && isExpression(location)) {
             return ScriptElementKind.parameterElement;
         }
-        const { flags } = symbol;
+        const flags = getCombinedLocalAndExportSymbolFlags(symbol);
         if (flags & SymbolFlags.Variable) {
             if (isFirstDeclarationOfSymbolParameter(symbol)) {
                 return ScriptElementKind.parameterElement;
@@ -96,7 +96,7 @@ namespace ts.SymbolDisplay {
         const displayParts: SymbolDisplayPart[] = [];
         let documentation: SymbolDisplayPart[];
         let tags: JSDocTagInfo[];
-        const symbolFlags = symbol.flags;
+        const symbolFlags = ts.getCombinedLocalAndExportSymbolFlags(symbol);
         let symbolKind = getSymbolKindOfConstructorPropertyMethodAccessorFunctionOrVar(typeChecker, symbol, location);
         let hasAddedSymbolInfo: boolean;
         const isThisExpression = location.kind === SyntaxKind.ThisKeyword && isExpression(location);
@@ -110,7 +110,7 @@ namespace ts.SymbolDisplay {
             }
 
             let signature: Signature;
-            type = isThisExpression ? typeChecker.getTypeAtLocation(location) : typeChecker.getTypeOfSymbolAtLocation(symbol, location);
+            type = isThisExpression ? typeChecker.getTypeAtLocation(location) : typeChecker.getTypeOfSymbolAtLocation(symbol.exportSymbol || symbol, location);
             if (type) {
                 if (location.parent && location.parent.kind === SyntaxKind.PropertyAccessExpression) {
                     const right = (<PropertyAccessExpression>location.parent).name;
@@ -198,7 +198,7 @@ namespace ts.SymbolDisplay {
                         hasAddedSymbolInfo = true;
                     }
                 }
-                else if ((isNameOfFunctionDeclaration(location) && !(symbol.flags & SymbolFlags.Accessor)) || // name of function declaration
+                else if ((isNameOfFunctionDeclaration(location) && !(symbolFlags & SymbolFlags.Accessor)) || // name of function declaration
                     (location.kind === SyntaxKind.ConstructorKeyword && location.parent.kind === SyntaxKind.Constructor)) { // At constructor keyword of constructor declaration
                     // get the signature from the declaration and write it
                     const functionDeclaration = <FunctionLike>location.parent;
@@ -429,7 +429,7 @@ namespace ts.SymbolDisplay {
         if (!documentation) {
             documentation = symbol.getDocumentationComment();
             tags = symbol.getJsDocTags();
-            if (documentation.length === 0 && symbol.flags & SymbolFlags.Property) {
+            if (documentation.length === 0 && symbolFlags & SymbolFlags.Property) {
                 // For some special property access expressions like `exports.foo = foo` or `module.exports.foo = foo`
                 // there documentation comments might be attached to the right hand side symbol of their declarations.
                 // The pattern of such special property access is that the parent symbol is the symbol of the file.
diff --git a/src/services/types.ts b/src/services/types.ts
index d440ecab28e..bfb8a80b769 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -164,7 +164,7 @@ namespace ts {
          * LS host can optionally implement these methods to support completions for module specifiers.
          * Without these methods, only completions for ambient modules will be provided.
          */
-        readDirectory?(path: string, extensions?: string[], exclude?: string[], include?: string[], depth?: number): string[];
+        readDirectory?(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
         readFile?(path: string, encoding?: string): string;
         fileExists?(path: string): boolean;
 
diff --git a/src/services/utilities.ts b/src/services/utilities.ts
index 5819b5a60ac..7fcfffc3ee2 100644
--- a/src/services/utilities.ts
+++ b/src/services/utilities.ts
@@ -1240,10 +1240,13 @@ namespace ts {
     }
 
     export function mapToDisplayParts(writeDisplayParts: (writer: DisplayPartsSymbolWriter) => void): SymbolDisplayPart[] {
-        writeDisplayParts(displayPartWriter);
-        const result = displayPartWriter.displayParts();
-        displayPartWriter.clear();
-        return result;
+        try {
+            writeDisplayParts(displayPartWriter);
+            return displayPartWriter.displayParts();
+        }
+        finally {
+            displayPartWriter.clear();
+        }
     }
 
     export function typeToDisplayParts(typechecker: TypeChecker, type: Type, enclosingDeclaration?: Node, flags?: TypeFormatFlags): SymbolDisplayPart[] {
diff --git a/tests/baselines/reference/checkJsdocTypeTag1.js b/tests/baselines/reference/checkJsdocTypeTag1.js
index e4586643c22..664a02c6047 100644
--- a/tests/baselines/reference/checkJsdocTypeTag1.js
+++ b/tests/baselines/reference/checkJsdocTypeTag1.js
@@ -28,7 +28,17 @@ x1(0);
 
 /** @type {function (number): number} */
 const x2 = (a) => a + 1;
-x2(0);
+x2(0);
+
+/**
+ * @type {object}
+ */
+var props = {};
+
+/**
+ * @type {Object}
+ */
+var props = {};
 
 //// [0.js]
 // @ts-check
@@ -54,3 +64,11 @@ x1(0);
 /** @type {function (number): number} */
 var x2 = function (a) { return a + 1; };
 x2(0);
+/**
+ * @type {object}
+ */
+var props = {};
+/**
+ * @type {Object}
+ */
+var props = {};
diff --git a/tests/baselines/reference/checkJsdocTypeTag1.symbols b/tests/baselines/reference/checkJsdocTypeTag1.symbols
index fff51499bc4..cd337f8b556 100644
--- a/tests/baselines/reference/checkJsdocTypeTag1.symbols
+++ b/tests/baselines/reference/checkJsdocTypeTag1.symbols
@@ -58,3 +58,15 @@ const x2 = (a) => a + 1;
 x2(0);
 >x2 : Symbol(x2, Decl(0.js, 28, 5))
 
+/**
+ * @type {object}
+ */
+var props = {};
+>props : Symbol(props, Decl(0.js, 34, 3), Decl(0.js, 39, 3))
+
+/**
+ * @type {Object}
+ */
+var props = {};
+>props : Symbol(props, Decl(0.js, 34, 3), Decl(0.js, 39, 3))
+
diff --git a/tests/baselines/reference/checkJsdocTypeTag1.types b/tests/baselines/reference/checkJsdocTypeTag1.types
index 898cf58806a..8e59a2cdb7c 100644
--- a/tests/baselines/reference/checkJsdocTypeTag1.types
+++ b/tests/baselines/reference/checkJsdocTypeTag1.types
@@ -86,3 +86,17 @@ x2(0);
 >x2 : (arg0: number) => number
 >0 : 0
 
+/**
+ * @type {object}
+ */
+var props = {};
+>props : any
+>{} : {}
+
+/**
+ * @type {Object}
+ */
+var props = {};
+>props : any
+>{} : {}
+
diff --git a/tests/baselines/reference/importCallExpressionShouldNotGetParen.js b/tests/baselines/reference/importCallExpressionShouldNotGetParen.js
new file mode 100644
index 00000000000..07539664aa3
--- /dev/null
+++ b/tests/baselines/reference/importCallExpressionShouldNotGetParen.js
@@ -0,0 +1,18 @@
+//// [importCallExpressionShouldNotGetParen.ts]
+const localeName = "zh-CN";
+import(`./locales/${localeName}.js`).then(bar => {
+    let x = bar;
+});
+
+import("./locales/" + localeName + ".js").then(bar => {
+    let x = bar;
+});
+
+//// [importCallExpressionShouldNotGetParen.js]
+const localeName = "zh-CN";
+import(`./locales/${localeName}.js`).then(bar => {
+    let x = bar;
+});
+import("./locales/" + localeName + ".js").then(bar => {
+    let x = bar;
+});
diff --git a/tests/baselines/reference/importCallExpressionShouldNotGetParen.symbols b/tests/baselines/reference/importCallExpressionShouldNotGetParen.symbols
new file mode 100644
index 00000000000..ace34efcf0d
--- /dev/null
+++ b/tests/baselines/reference/importCallExpressionShouldNotGetParen.symbols
@@ -0,0 +1,27 @@
+=== tests/cases/conformance/dynamicImport/importCallExpressionShouldNotGetParen.ts ===
+const localeName = "zh-CN";
+>localeName : Symbol(localeName, Decl(importCallExpressionShouldNotGetParen.ts, 0, 5))
+
+import(`./locales/${localeName}.js`).then(bar => {
+>import(`./locales/${localeName}.js`).then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>localeName : Symbol(localeName, Decl(importCallExpressionShouldNotGetParen.ts, 0, 5))
+>then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>bar : Symbol(bar, Decl(importCallExpressionShouldNotGetParen.ts, 1, 42))
+
+    let x = bar;
+>x : Symbol(x, Decl(importCallExpressionShouldNotGetParen.ts, 2, 7))
+>bar : Symbol(bar, Decl(importCallExpressionShouldNotGetParen.ts, 1, 42))
+
+});
+
+import("./locales/" + localeName + ".js").then(bar => {
+>import("./locales/" + localeName + ".js").then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>localeName : Symbol(localeName, Decl(importCallExpressionShouldNotGetParen.ts, 0, 5))
+>then : Symbol(Promise.then, Decl(lib.es5.d.ts, --, --))
+>bar : Symbol(bar, Decl(importCallExpressionShouldNotGetParen.ts, 5, 47))
+
+    let x = bar;
+>x : Symbol(x, Decl(importCallExpressionShouldNotGetParen.ts, 6, 7))
+>bar : Symbol(bar, Decl(importCallExpressionShouldNotGetParen.ts, 5, 47))
+
+});
diff --git a/tests/baselines/reference/importCallExpressionShouldNotGetParen.types b/tests/baselines/reference/importCallExpressionShouldNotGetParen.types
new file mode 100644
index 00000000000..3e4001dc5b7
--- /dev/null
+++ b/tests/baselines/reference/importCallExpressionShouldNotGetParen.types
@@ -0,0 +1,39 @@
+=== tests/cases/conformance/dynamicImport/importCallExpressionShouldNotGetParen.ts ===
+const localeName = "zh-CN";
+>localeName : "zh-CN"
+>"zh-CN" : "zh-CN"
+
+import(`./locales/${localeName}.js`).then(bar => {
+>import(`./locales/${localeName}.js`).then(bar => {    let x = bar;}) : Promise<void>
+>import(`./locales/${localeName}.js`).then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>import(`./locales/${localeName}.js`) : Promise<any>
+>`./locales/${localeName}.js` : string
+>localeName : "zh-CN"
+>then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>bar => {    let x = bar;} : (bar: any) => void
+>bar : any
+
+    let x = bar;
+>x : any
+>bar : any
+
+});
+
+import("./locales/" + localeName + ".js").then(bar => {
+>import("./locales/" + localeName + ".js").then(bar => {    let x = bar;}) : Promise<void>
+>import("./locales/" + localeName + ".js").then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>import("./locales/" + localeName + ".js") : Promise<any>
+>"./locales/" + localeName + ".js" : string
+>"./locales/" + localeName : string
+>"./locales/" : "./locales/"
+>localeName : "zh-CN"
+>".js" : ".js"
+>then : <TResult1 = any, TResult2 = never>(onfulfilled?: (value: any) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => Promise<TResult1 | TResult2>
+>bar => {    let x = bar;} : (bar: any) => void
+>bar : any
+
+    let x = bar;
+>x : any
+>bar : any
+
+});
diff --git a/tests/baselines/reference/importCallExpressionWithTypeArgument.js b/tests/baselines/reference/importCallExpressionWithTypeArgument.js
index d8690cf5d75..cdea13b2fac 100644
--- a/tests/baselines/reference/importCallExpressionWithTypeArgument.js
+++ b/tests/baselines/reference/importCallExpressionWithTypeArgument.js
@@ -18,5 +18,5 @@ function foo() { return "foo"; }
 exports.foo = foo;
 //// [1.js]
 "use strict";
-var p1 = (import)("./0"); // error
-var p2 = (import)("./0"); // error
+var p1 = Promise.resolve().then(function () { return require("./0"); }); // error
+var p2 = Promise.resolve().then(function () { return require("./0"); }); // error
diff --git a/tests/baselines/reference/jsdocTypeTag.js b/tests/baselines/reference/jsdocTypeTag.js
index ff92f0f0474..4789b1d26af 100644
--- a/tests/baselines/reference/jsdocTypeTag.js
+++ b/tests/baselines/reference/jsdocTypeTag.js
@@ -55,6 +55,9 @@ var nullable;
 /** @type {Object} */
 var Obj;
 
+/** @type {object} */
+var obj;
+
 /** @type {Function} */
 var Func;
 
@@ -77,6 +80,7 @@ var P: Promise<any>;
 var p: Promise<any>;
 var nullable: number | null;
 var Obj: any;
+var obj: any;
 var Func: Function;
 
 
@@ -117,6 +121,8 @@ var p;
 var nullable;
 /** @type {Object} */
 var Obj;
+/** @type {object} */
+var obj;
 /** @type {Function} */
 var Func;
 //// [b.js]
@@ -138,4 +144,5 @@ var P;
 var p;
 var nullable;
 var Obj;
+var obj;
 var Func;
diff --git a/tests/baselines/reference/jsdocTypeTag.symbols b/tests/baselines/reference/jsdocTypeTag.symbols
index 5931c1cfe53..77df5b119f4 100644
--- a/tests/baselines/reference/jsdocTypeTag.symbols
+++ b/tests/baselines/reference/jsdocTypeTag.symbols
@@ -71,9 +71,13 @@ var nullable;
 var Obj;
 >Obj : Symbol(Obj, Decl(a.js, 52, 3), Decl(b.ts, 17, 3))
 
+/** @type {object} */
+var obj;
+>obj : Symbol(obj, Decl(a.js, 55, 3), Decl(b.ts, 18, 3))
+
 /** @type {Function} */
 var Func;
->Func : Symbol(Func, Decl(a.js, 55, 3), Decl(b.ts, 18, 3))
+>Func : Symbol(Func, Decl(a.js, 58, 3), Decl(b.ts, 19, 3))
 
 === tests/cases/conformance/jsdoc/b.ts ===
 var S: string;
@@ -132,7 +136,10 @@ var nullable: number | null;
 var Obj: any;
 >Obj : Symbol(Obj, Decl(a.js, 52, 3), Decl(b.ts, 17, 3))
 
+var obj: any;
+>obj : Symbol(obj, Decl(a.js, 55, 3), Decl(b.ts, 18, 3))
+
 var Func: Function;
->Func : Symbol(Func, Decl(a.js, 55, 3), Decl(b.ts, 18, 3))
+>Func : Symbol(Func, Decl(a.js, 58, 3), Decl(b.ts, 19, 3))
 >Function : Symbol(Function, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
 
diff --git a/tests/baselines/reference/jsdocTypeTag.types b/tests/baselines/reference/jsdocTypeTag.types
index f3e7dfad634..9813a186a00 100644
--- a/tests/baselines/reference/jsdocTypeTag.types
+++ b/tests/baselines/reference/jsdocTypeTag.types
@@ -71,6 +71,10 @@ var nullable;
 var Obj;
 >Obj : any
 
+/** @type {object} */
+var obj;
+>obj : any
+
 /** @type {Function} */
 var Func;
 >Func : Function
@@ -135,6 +139,9 @@ var nullable: number | null;
 var Obj: any;
 >Obj : any
 
+var obj: any;
+>obj : any
+
 var Func: Function;
 >Func : Function
 >Function : Function
diff --git a/tests/baselines/reference/mergedDeclarationExports.errors.txt b/tests/baselines/reference/mergedDeclarationExports.errors.txt
new file mode 100644
index 00000000000..f7a09ad4bc6
--- /dev/null
+++ b/tests/baselines/reference/mergedDeclarationExports.errors.txt
@@ -0,0 +1,44 @@
+tests/cases/compiler/mergedDeclarationExports.ts(13,11): error TS2395: Individual declarations in merged declaration 'c' must be all exported or all local.
+tests/cases/compiler/mergedDeclarationExports.ts(14,18): error TS2395: Individual declarations in merged declaration 'c' must be all exported or all local.
+tests/cases/compiler/mergedDeclarationExports.ts(17,11): error TS2395: Individual declarations in merged declaration 'd' must be all exported or all local.
+tests/cases/compiler/mergedDeclarationExports.ts(18,14): error TS2395: Individual declarations in merged declaration 'd' must be all exported or all local.
+tests/cases/compiler/mergedDeclarationExports.ts(21,11): error TS2395: Individual declarations in merged declaration 'N' must be all exported or all local.
+tests/cases/compiler/mergedDeclarationExports.ts(22,18): error TS2395: Individual declarations in merged declaration 'N' must be all exported or all local.
+
+
+==== tests/cases/compiler/mergedDeclarationExports.ts (6 errors) ====
+    // OK -- one is type, one is value
+    interface b {}
+    export const b = 1;
+    
+    // OK -- one is a type, one is a namespace, one is a value.
+    type t = 0;
+    namespace t { interface I {} }
+    export const t = 0;
+    
+    // Should get errors if they have some meaning in common.
+    
+    // both types
+    interface c {}
+              ~
+!!! error TS2395: Individual declarations in merged declaration 'c' must be all exported or all local.
+    export interface c {}
+                     ~
+!!! error TS2395: Individual declarations in merged declaration 'c' must be all exported or all local.
+    
+    // both types (class is also value, but that doesn't matter)
+    interface d {}
+              ~
+!!! error TS2395: Individual declarations in merged declaration 'd' must be all exported or all local.
+    export class d {}
+                 ~
+!!! error TS2395: Individual declarations in merged declaration 'd' must be all exported or all local.
+    
+    // both namespaces
+    namespace N { }
+              ~
+!!! error TS2395: Individual declarations in merged declaration 'N' must be all exported or all local.
+    export namespace N {}
+                     ~
+!!! error TS2395: Individual declarations in merged declaration 'N' must be all exported or all local.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/mergedDeclarationExports.js b/tests/baselines/reference/mergedDeclarationExports.js
new file mode 100644
index 00000000000..068f7b683c3
--- /dev/null
+++ b/tests/baselines/reference/mergedDeclarationExports.js
@@ -0,0 +1,36 @@
+//// [mergedDeclarationExports.ts]
+// OK -- one is type, one is value
+interface b {}
+export const b = 1;
+
+// OK -- one is a type, one is a namespace, one is a value.
+type t = 0;
+namespace t { interface I {} }
+export const t = 0;
+
+// Should get errors if they have some meaning in common.
+
+// both types
+interface c {}
+export interface c {}
+
+// both types (class is also value, but that doesn't matter)
+interface d {}
+export class d {}
+
+// both namespaces
+namespace N { }
+export namespace N {}
+
+
+//// [mergedDeclarationExports.js]
+"use strict";
+exports.__esModule = true;
+exports.b = 1;
+exports.t = 0;
+var d = (function () {
+    function d() {
+    }
+    return d;
+}());
+exports.d = d;
diff --git a/tests/baselines/reference/parseErrorIncorrectReturnToken.errors.txt b/tests/baselines/reference/parseErrorIncorrectReturnToken.errors.txt
new file mode 100644
index 00000000000..2cf728848e4
--- /dev/null
+++ b/tests/baselines/reference/parseErrorIncorrectReturnToken.errors.txt
@@ -0,0 +1,37 @@
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(2,17): error TS1005: ':' expected.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(4,22): error TS1005: '=>' expected.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(4,24): error TS2693: 'string' only refers to a type, but is being used as a value here.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(9,18): error TS1005: '{' expected.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(9,21): error TS2693: 'string' only refers to a type, but is being used as a value here.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(9,28): error TS1005: ';' expected.
+tests/cases/compiler/parseErrorIncorrectReturnToken.ts(12,1): error TS1128: Declaration or statement expected.
+
+
+==== tests/cases/compiler/parseErrorIncorrectReturnToken.ts (7 errors) ====
+    type F1 = {
+        (n: number) => string; // should be : not =>
+                    ~~
+!!! error TS1005: ':' expected.
+    }
+    type F2 = (n: number): string; // should be => not :
+                         ~
+!!! error TS1005: '=>' expected.
+                           ~~~~~~
+!!! error TS2693: 'string' only refers to a type, but is being used as a value here.
+    
+    // doesn't work in non-type contexts, where the return type is optional
+    let f = (n: number) => string => n.toString();
+    let o = {
+        m(n: number) => string {
+                     ~~
+!!! error TS1005: '{' expected.
+                        ~~~~~~
+!!! error TS2693: 'string' only refers to a type, but is being used as a value here.
+                               ~
+!!! error TS1005: ';' expected.
+            return n.toString();
+        }
+    };
+    ~
+!!! error TS1128: Declaration or statement expected.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/parseErrorIncorrectReturnToken.js b/tests/baselines/reference/parseErrorIncorrectReturnToken.js
new file mode 100644
index 00000000000..3d2a1d17a4a
--- /dev/null
+++ b/tests/baselines/reference/parseErrorIncorrectReturnToken.js
@@ -0,0 +1,27 @@
+//// [parseErrorIncorrectReturnToken.ts]
+type F1 = {
+    (n: number) => string; // should be : not =>
+}
+type F2 = (n: number): string; // should be => not :
+
+// doesn't work in non-type contexts, where the return type is optional
+let f = (n: number) => string => n.toString();
+let o = {
+    m(n: number) => string {
+        return n.toString();
+    }
+};
+
+
+//// [parseErrorIncorrectReturnToken.js]
+string; // should be => not :
+// doesn't work in non-type contexts, where the return type is optional
+var f = function (n) { return function (string) { return n.toString(); }; };
+var o = {
+    m: function (n) { }
+};
+string;
+{
+    return n.toString();
+}
+;
diff --git a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json
index 772c218eb4e..b79b4f0f181 100644
--- a/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Default initialized TSConfig/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json
index 7a9b895636c..ecbaaf9d961 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with boolean value compiler options/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json
index 70d5ed9d738..321547908d9 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with enum value compiler options/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json
index 914b9d99d1b..ec0ac6e4c32 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with files options/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json
index 50bd28442bb..655ece4d6d0 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option value/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     "lib": ["es5","es2015.promise"],          /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json
index 772c218eb4e..b79b4f0f181 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with incorrect compiler option/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json
index afae7193d58..81b1636b8cb 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options with enum value/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     "lib": ["es5","es2015.core"],             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json
index a87fe9c5206..66acf6df045 100644
--- a/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
+++ b/tests/baselines/reference/tsConfig/Initialized TSConfig with list compiler options/tsconfig.json	
@@ -2,7 +2,7 @@
   "compilerOptions": {
     /* Basic Options */
     "target": "es5",                          /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017', or 'ESNEXT'. */
-    "module": "commonjs",                     /* Specify module code generation: 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
+    "module": "commonjs",                     /* Specify module code generation: 'none', commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */
     // "lib": [],                             /* Specify library files to be included in the compilation:  */
     // "allowJs": true,                       /* Allow javascript files to be compiled. */
     // "checkJs": true,                       /* Report errors in .js files. */
diff --git a/tests/cases/compiler/mergedDeclarationExports.ts b/tests/cases/compiler/mergedDeclarationExports.ts
new file mode 100644
index 00000000000..a27f1186e9f
--- /dev/null
+++ b/tests/cases/compiler/mergedDeclarationExports.ts
@@ -0,0 +1,22 @@
+// OK -- one is type, one is value
+interface b {}
+export const b = 1;
+
+// OK -- one is a type, one is a namespace, one is a value.
+type t = 0;
+namespace t { interface I {} }
+export const t = 0;
+
+// Should get errors if they have some meaning in common.
+
+// both types
+interface c {}
+export interface c {}
+
+// both types (class is also value, but that doesn't matter)
+interface d {}
+export class d {}
+
+// both namespaces
+namespace N { }
+export namespace N {}
diff --git a/tests/cases/compiler/parseErrorIncorrectReturnToken.ts b/tests/cases/compiler/parseErrorIncorrectReturnToken.ts
new file mode 100644
index 00000000000..0f45c38a3b1
--- /dev/null
+++ b/tests/cases/compiler/parseErrorIncorrectReturnToken.ts
@@ -0,0 +1,13 @@
+
+type F1 = {
+    (n: number) => string; // should be : not =>
+}
+type F2 = (n: number): string; // should be => not :
+
+// doesn't work in non-type contexts, where the return type is optional
+let f = (n: number) => string => n.toString();
+let o = {
+    m(n: number) => string {
+        return n.toString();
+    }
+};
diff --git a/tests/cases/conformance/dynamicImport/importCallExpressionShouldNotGetParen.ts b/tests/cases/conformance/dynamicImport/importCallExpressionShouldNotGetParen.ts
new file mode 100644
index 00000000000..6734e888a5e
--- /dev/null
+++ b/tests/cases/conformance/dynamicImport/importCallExpressionShouldNotGetParen.ts
@@ -0,0 +1,11 @@
+// @module: esnext
+// @target: es6
+// @noImplicitAny: true
+const localeName = "zh-CN";
+import(`./locales/${localeName}.js`).then(bar => {
+    let x = bar;
+});
+
+import("./locales/" + localeName + ".js").then(bar => {
+    let x = bar;
+});
\ No newline at end of file
diff --git a/tests/cases/conformance/jsdoc/checkJsdocTypeTag1.ts b/tests/cases/conformance/jsdoc/checkJsdocTypeTag1.ts
index fbed9d83b22..4dc7b934571 100644
--- a/tests/cases/conformance/jsdoc/checkJsdocTypeTag1.ts
+++ b/tests/cases/conformance/jsdoc/checkJsdocTypeTag1.ts
@@ -31,4 +31,14 @@ x1(0);
 
 /** @type {function (number): number} */
 const x2 = (a) => a + 1;
-x2(0);
\ No newline at end of file
+x2(0);
+
+/**
+ * @type {object}
+ */
+var props = {};
+
+/**
+ * @type {Object}
+ */
+var props = {};
\ No newline at end of file
diff --git a/tests/cases/conformance/jsdoc/jsdocTypeTag.ts b/tests/cases/conformance/jsdoc/jsdocTypeTag.ts
index d566c61e185..1c19771a3cd 100644
--- a/tests/cases/conformance/jsdoc/jsdocTypeTag.ts
+++ b/tests/cases/conformance/jsdoc/jsdocTypeTag.ts
@@ -57,6 +57,9 @@ var nullable;
 /** @type {Object} */
 var Obj;
 
+/** @type {object} */
+var obj;
+
 /** @type {Function} */
 var Func;
 
@@ -79,4 +82,5 @@ var P: Promise<any>;
 var p: Promise<any>;
 var nullable: number | null;
 var Obj: any;
+var obj: any;
 var Func: Function;
diff --git a/tests/cases/fourslash/commentsExternalModules.ts b/tests/cases/fourslash/commentsExternalModules.ts
index 4f1a82bc0ce..582c6a31357 100644
--- a/tests/cases/fourslash/commentsExternalModules.ts
+++ b/tests/cases/fourslash/commentsExternalModules.ts
@@ -35,7 +35,7 @@ goTo.file("commentsExternalModules_file0.ts");
 verify.quickInfoAt("1", "namespace m1", "Namespace comment");
 
 goTo.marker('2');
-verify.completionListContains("b", "var m1.b: number", "b's comment");
+verify.completionListContains("b", "var b: number", "b's comment");
 verify.completionListContains("foo", "function foo(): number", "foo's comment");
 
 goTo.marker('3');
diff --git a/tests/cases/fourslash/commentsModules.ts b/tests/cases/fourslash/commentsModules.ts
index c0b8ccc5731..0205b144f16 100644
--- a/tests/cases/fourslash/commentsModules.ts
+++ b/tests/cases/fourslash/commentsModules.ts
@@ -99,7 +99,7 @@
 verify.quickInfoAt("1", "namespace m1", "Namespace comment");
 
 goTo.marker('2');
-verify.completionListContains("b", "var m1.b: number", "b's comment");
+verify.completionListContains("b", "var b: number", "b's comment");
 verify.completionListContains("foo", "function foo(): number", "foo's comment");
 
 goTo.marker('3');
diff --git a/tests/cases/fourslash/completionListForUnicodeEscapeName.ts b/tests/cases/fourslash/completionListForUnicodeEscapeName.ts
index afafd18a58b..9ada3e35961 100644
--- a/tests/cases/fourslash/completionListForUnicodeEscapeName.ts
+++ b/tests/cases/fourslash/completionListForUnicodeEscapeName.ts
@@ -6,8 +6,8 @@
 /////*3*/
 
 goTo.marker("0");
-verify.not.completionListContains("B");
-verify.not.completionListContains("\u0042");
+verify.completionListContains("B");
+verify.completionListContains("\u0042");
 
 goTo.marker("2");
 verify.not.completionListContains("C");
@@ -18,10 +18,9 @@ verify.not.completionListContains("A");
 verify.not.completionListContains("\u0041");
 
 goTo.marker("3");
-verify.not.completionListContains("B");
-verify.not.completionListContains("\u0042");
-verify.not.completionListContains("A");
-verify.not.completionListContains("\u0041");
-verify.not.completionListContains("C");
-verify.not.completionListContains("\u0043");
-
+verify.completionListContains("B");
+verify.completionListContains("\u0042");
+verify.completionListContains("A");
+verify.completionListContains("\u0041");
+verify.completionListContains("C");
+verify.completionListContains("\u0043");
diff --git a/tests/cases/fourslash/completionListInScope_doesNotIncludeAugmentations.ts b/tests/cases/fourslash/completionListInScope_doesNotIncludeAugmentations.ts
new file mode 100644
index 00000000000..de70890c4c6
--- /dev/null
+++ b/tests/cases/fourslash/completionListInScope_doesNotIncludeAugmentations.ts
@@ -0,0 +1,13 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /a.ts
+////import * as self from "./a";
+////
+////declare module "a" {
+////    export const a: number;
+////}
+////
+/////**/
+
+goTo.marker();
+verify.not.completionListContains("a");
diff --git a/tests/cases/fourslash/completionListInvalidMemberNames.ts b/tests/cases/fourslash/completionListInvalidMemberNames.ts
index 95abe70415c..e0a65bfca4d 100644
--- a/tests/cases/fourslash/completionListInvalidMemberNames.ts
+++ b/tests/cases/fourslash/completionListInvalidMemberNames.ts
@@ -19,6 +19,7 @@ verify.completionListContains("bar");
 verify.completionListContains("break");
 verify.completionListContains("any");
 verify.completionListContains("$");
+verify.completionListContains("b");
 
 // Nothing else should show up
-verify.completionListCount(4);
+verify.completionListCount(5);
diff --git a/tests/cases/fourslash/completionListOnAliases2.ts b/tests/cases/fourslash/completionListOnAliases2.ts
index 63f895f276e..5933fbe89d4 100644
--- a/tests/cases/fourslash/completionListOnAliases2.ts
+++ b/tests/cases/fourslash/completionListOnAliases2.ts
@@ -41,19 +41,16 @@ function getVerify(isTypeLocation?: boolean) {
         verifyValueOrType: verify
     };
 }
-function typeLocationVerify(valueMarker: string, verify: (typeMarker: string) => void) {
-    verify(valueMarker + "Type");
-    return valueMarker;
-}
 
 function verifyModuleM(marker: string) {
-    const isTypeLocation = marker.indexOf("Type") !== -1;
-    const { verifyValue, verifyType, verifyValueOrType } = getVerify(isTypeLocation);
-    if (!isTypeLocation) {
-        marker = typeLocationVerify(marker, verifyModuleM);
-    }
+    verifyModuleMWorker(marker, /*isTypeLocation*/ false);
+    verifyModuleMWorker(`${marker}Type`, /*isTypeLocation*/ true);
+}
+
+function verifyModuleMWorker(marker: string, isTypeLocation: boolean): void {
     goTo.marker(marker);
 
+    const { verifyValue, verifyType, verifyValueOrType } = getVerify(isTypeLocation);
     verifyType.completionListContains("I");
     verifyValueOrType.completionListContains("C");
     verifyValueOrType.completionListContains("E");
@@ -63,8 +60,9 @@ function verifyModuleM(marker: string) {
     verifyValueOrType.completionListContains("A");
 }
 
-
 // Module m
+goTo.marker("1");
+verify.completionListContains("A");
 verifyModuleM("1");
 
 // Class C
diff --git a/tests/cases/fourslash/completionListWithModulesInsideModuleScope.ts b/tests/cases/fourslash/completionListWithModulesInsideModuleScope.ts
index 5d950a5d620..cb6bf283b69 100644
--- a/tests/cases/fourslash/completionListWithModulesInsideModuleScope.ts
+++ b/tests/cases/fourslash/completionListWithModulesInsideModuleScope.ts
@@ -232,6 +232,7 @@
 interface GotoMarkVerifyOptions {
     isClassScope?: boolean;
     isTypeLocation?: boolean;
+    insideMod1?: boolean;
 }
 
 function getVerify(isTypeLocation: boolean) {
@@ -243,30 +244,30 @@ function getVerify(isTypeLocation: boolean) {
     };
 }
 
-function goToMarkAndGeneralVerify(marker: string, { isClassScope, isTypeLocation }: GotoMarkVerifyOptions = {})
-{
+function goToMarkAndGeneralVerify(marker: string, { isClassScope, isTypeLocation, insideMod1 }: GotoMarkVerifyOptions = {}) {
     goTo.marker(marker);
 
+    const mod1Dot = insideMod1 ? "" : "mod1.";
     const verifyValueInModule = isClassScope || isTypeLocation ? verify.not : verify;
     const verifyValueOrTypeInModule = isClassScope ? verify.not : verify;
     const verifyTypeInModule = isTypeLocation ? verify : verify.not;
     verifyValueInModule.completionListContains('mod1var', 'var mod1var: number');
     verifyValueInModule.completionListContains('mod1fn', 'function mod1fn(): void');
-    verifyValueInModule.completionListContains('mod1evar', 'var mod1.mod1evar: number');
-    verifyValueInModule.completionListContains('mod1efn', 'function mod1.mod1efn(): void');
-    verifyValueInModule.completionListContains('mod1eexvar', 'var mod1.mod1eexvar: number');
+    verifyValueInModule.completionListContains('mod1evar', `var ${mod1Dot}mod1evar: number`);
+    verifyValueInModule.completionListContains('mod1efn', `function ${mod1Dot}mod1efn(): void`);
+    verifyValueInModule.completionListContains('mod1eexvar', `var mod1.mod1eexvar: number`);
     verifyValueInModule.completionListContains('mod3', 'namespace mod3');
     verifyValueInModule.completionListContains('shwvar', 'var shwvar: number');
     verifyValueInModule.completionListContains('shwfn', 'function shwfn(): void');
 
     verifyTypeInModule.completionListContains('mod1int', 'interface mod1int');
-    verifyTypeInModule.completionListContains('mod1eint', 'interface mod1.mod1eint');
+    verifyTypeInModule.completionListContains('mod1eint', `interface ${mod1Dot}mod1eint`);
     verifyTypeInModule.completionListContains('shwint', 'interface shwint');
 
     verifyValueOrTypeInModule.completionListContains('mod1cls', 'class mod1cls');
     verifyValueOrTypeInModule.completionListContains('mod1mod', 'namespace mod1mod');
-    verifyValueOrTypeInModule.completionListContains('mod1ecls', 'class mod1.mod1ecls');
-    verifyValueOrTypeInModule.completionListContains('mod1emod', 'namespace mod1.mod1emod');
+    verifyValueOrTypeInModule.completionListContains('mod1ecls', `class ${mod1Dot}mod1ecls`);
+    verifyValueOrTypeInModule.completionListContains('mod1emod', `namespace ${mod1Dot}mod1emod`);
     verifyValueOrTypeInModule.completionListContains('mod2', 'namespace mod2');
     verifyValueOrTypeInModule.completionListContains('shwcls', 'class shwcls');
 
@@ -295,12 +296,12 @@ function goToMarkAndGeneralVerify(marker: string, { isClassScope, isTypeLocation
 }
 
 // from mod1
-goToMarkAndGeneralVerify('mod1');
+goToMarkAndGeneralVerify('mod1', { insideMod1: true });
 // from mod1 in type position
-goToMarkAndGeneralVerify('mod1Type', { isTypeLocation: true });
+goToMarkAndGeneralVerify('mod1Type', { isTypeLocation: true, insideMod1: true });
 
 // from function in mod1
-goToMarkAndGeneralVerify('function');
+goToMarkAndGeneralVerify('function', { insideMod1: true });
 verify.completionListContains('bar', '(local var) bar: number');
 verify.completionListContains('foob', '(local function) foob(): void');
 
@@ -310,34 +311,34 @@ goToMarkAndGeneralVerify('class', { isClassScope: true });
 //verify.not.completionListContains('ceVar');
 
 // from interface in mod1
-goToMarkAndGeneralVerify('interface');
+goToMarkAndGeneralVerify('interface', { insideMod1: true });
 
 // from namespace in mod1
 verifyNamespaceInMod1('namespace');
 verifyNamespaceInMod1('namespaceType', /*isTypeLocation*/ true);
 
 function verifyNamespaceInMod1(marker: string, isTypeLocation?: boolean) {
-    goToMarkAndGeneralVerify(marker, { isTypeLocation });
+    goToMarkAndGeneralVerify(marker, { isTypeLocation, insideMod1: true });
 
     const { verifyValue, verifyType, verifyValueOrType, verifyNotValueOrType } = getVerify(isTypeLocation);
 
     verifyValue.completionListContains('m1X', 'var m1X: number');
     verifyValue.completionListContains('m1Func', 'function m1Func(): void');
-    verifyValue.completionListContains('m1eX', 'var mod1mod.m1eX: number');
-    verifyValue.completionListContains('m1eFunc', 'function mod1mod.m1eFunc(): void');
+    verifyValue.completionListContains('m1eX', 'var m1eX: number');
+    verifyValue.completionListContains('m1eFunc', 'function m1eFunc(): void');
 
     verifyType.completionListContains('m1Int', 'interface m1Int');
-    verifyType.completionListContains('m1eInt', 'interface mod1mod.m1eInt');
+    verifyType.completionListContains('m1eInt', 'interface m1eInt');
 
     verifyValueOrType.completionListContains('m1Class', 'class m1Class');
-    verifyValueOrType.completionListContains('m1eClass', 'class mod1mod.m1eClass');
+    verifyValueOrType.completionListContains('m1eClass', 'class m1eClass');
 
     verifyNotValueOrType.completionListContains('m1Mod', 'namespace m1Mod');
-    verifyNotValueOrType.completionListContains('m1eMod', 'namespace mod1mod.m1eMod');
+    verifyNotValueOrType.completionListContains('m1eMod', 'namespace m1eMod');
 }
 
 // from exported function in mod1
-goToMarkAndGeneralVerify('exportedFunction');
+goToMarkAndGeneralVerify('exportedFunction', { insideMod1: true });
 verify.completionListContains('bar', '(local var) bar: number');
 verify.completionListContains('foob', '(local function) foob(): void');
 
@@ -347,27 +348,27 @@ verify.not.completionListContains('ceFunc');
 verify.not.completionListContains('ceVar');
 
 // from exported interface in mod1
-goToMarkAndGeneralVerify('exportedInterface');
+goToMarkAndGeneralVerify('exportedInterface', { insideMod1: true });
 
 // from exported namespace in mod1
 verifyExportedNamespace('exportedNamespace');
 verifyExportedNamespace('exportedNamespaceType', /*isTypeLocation*/ true);
 function verifyExportedNamespace(marker: string, isTypeLocation?: boolean) {
-    goToMarkAndGeneralVerify(marker, { isTypeLocation });
+    goToMarkAndGeneralVerify(marker, { isTypeLocation, insideMod1: true });
     const { verifyValue, verifyType, verifyValueOrType, verifyNotValueOrType } = getVerify(isTypeLocation);
     verifyValue.completionListContains('mX', 'var mX: number');
     verifyValue.completionListContains('mFunc', 'function mFunc(): void');
-    verifyValue.completionListContains('meX', 'var mod1.mod1emod.meX: number');
-    verifyValue.completionListContains('meFunc', 'function mod1.mod1emod.meFunc(): void');
+    verifyValue.completionListContains('meX', 'var meX: number');
+    verifyValue.completionListContains('meFunc', 'function meFunc(): void');
 
     verifyType.completionListContains('mInt', 'interface mInt');
-    verifyType.completionListContains('meInt', 'interface mod1.mod1emod.meInt');
+    verifyType.completionListContains('meInt', 'interface meInt');
 
     verifyValueOrType.completionListContains('mClass', 'class mClass');
-    verifyValueOrType.completionListContains('meClass', 'class mod1.mod1emod.meClass');
+    verifyValueOrType.completionListContains('meClass', 'class meClass');
 
     verifyNotValueOrType.completionListContains('mMod', 'namespace mMod');
-    verifyNotValueOrType.completionListContains('meMod', 'namespace mod1.mod1emod.meMod');
+    verifyNotValueOrType.completionListContains('meMod', 'namespace meMod');
 }
 
 // from extended namespace
@@ -380,7 +381,7 @@ function verifyExtendedNamespace(marker: string, isTypeLocation?: boolean) {
 
     verifyValue.completionListContains('mod1evar', 'var mod1.mod1evar: number');
     verifyValue.completionListContains('mod1efn', 'function mod1.mod1efn(): void');
-    verifyValue.completionListContains('mod1eexvar', 'var mod1.mod1eexvar: number');
+    verifyValue.completionListContains('mod1eexvar', 'var mod1eexvar: number');
     verifyValue.completionListContains('mod3', 'namespace mod3');
     verifyValue.completionListContains('shwvar', 'var shwvar: number');
     verifyValue.completionListContains('shwfn', 'function shwfn(): void');
@@ -415,4 +416,4 @@ function verifyExtendedNamespace(marker: string, isTypeLocation?: boolean) {
     verify.not.completionListContains('sivar');
     verify.not.completionListContains('sifn');
     verify.not.completionListContains('mod2eexvar');
-}
\ No newline at end of file
+}
diff --git a/tests/cases/fourslash/completionsDefaultExport.ts b/tests/cases/fourslash/completionsDefaultExport.ts
new file mode 100644
index 00000000000..82bcefac403
--- /dev/null
+++ b/tests/cases/fourslash/completionsDefaultExport.ts
@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+// @Filename: /a.ts
+////export default function f() {}
+
+// @Filename: /b.ts
+////import * as a from "./a";
+////a./**/;
+
+goTo.marker();
+verify.completionListContains("default", "function f(): void");
diff --git a/tests/cases/fourslash/completionsNamespaceMergedWithClass.ts b/tests/cases/fourslash/completionsNamespaceMergedWithClass.ts
new file mode 100644
index 00000000000..414a233bdb1
--- /dev/null
+++ b/tests/cases/fourslash/completionsNamespaceMergedWithClass.ts
@@ -0,0 +1,21 @@
+/// <reference path='fourslash.ts'/>
+
+////class C {
+////    static m() { }
+////}
+////
+////class D extends C {}
+////namespace D {
+////    export type T = number;
+////}
+////
+////let x: D./*type*/;
+////D./*value*/
+
+goTo.marker("type");
+verify.completionListContains("T");
+verify.not.completionListContains("m");
+
+goTo.marker("value");
+verify.not.completionListContains("T");
+verify.completionListContains("m");
diff --git a/tests/cases/fourslash/completionsNamespaceMergedWithObject.ts b/tests/cases/fourslash/completionsNamespaceMergedWithObject.ts
new file mode 100644
index 00000000000..cd799981ee7
--- /dev/null
+++ b/tests/cases/fourslash/completionsNamespaceMergedWithObject.ts
@@ -0,0 +1,16 @@
+/// <reference path='fourslash.ts'/>
+
+////namespace N {
+////    export type T = number;
+////}
+////const N = { m() {} };
+////let x: N./*type*/;
+////N./*value*/;
+
+goTo.marker("type");
+verify.completionListContains("T");
+verify.not.completionListContains("m");
+
+goTo.marker("value");
+verify.not.completionListContains("T");
+verify.completionListContains("m");
diff --git a/tests/cases/fourslash/exportDefaultFunction.ts b/tests/cases/fourslash/exportDefaultFunction.ts
index 859d9641acd..42ddced6994 100644
--- a/tests/cases/fourslash/exportDefaultFunction.ts
+++ b/tests/cases/fourslash/exportDefaultFunction.ts
@@ -9,4 +9,4 @@ goTo.marker('1');
 verify.completionListContains("func", "function func(): void", /*documentation*/ undefined, "function");
 
 goTo.marker('2');
-verify.completionListContains("func", "function func(): void", /*documentation*/ undefined, "function");
\ No newline at end of file
+verify.completionListContains("func", "function func(): void", /*documentation*/ undefined, "function");
diff --git a/tests/cases/fourslash/findAllRefsForDefaultExport02.ts b/tests/cases/fourslash/findAllRefsForDefaultExport02.ts
index 558bbd2de85..6ad8b29b5ce 100644
--- a/tests/cases/fourslash/findAllRefsForDefaultExport02.ts
+++ b/tests/cases/fourslash/findAllRefsForDefaultExport02.ts
@@ -15,8 +15,9 @@
 const ranges = test.ranges();
 const [r0, r1, r2, r3, r4] = ranges;
 const fnRanges = [r0, r1, r2, r3];
-verify.singleReferenceGroup("function DefaultExportedFunction(): () => typeof DefaultExportedFunction", fnRanges);
+const fn = "function DefaultExportedFunction(): () => typeof DefaultExportedFunction";
+verify.singleReferenceGroup(fn, fnRanges);
 
 // The namespace and function do not merge,
 // so the namespace should be all alone.
-verify.singleReferenceGroup("namespace DefaultExportedFunction", [r4]);
+verify.singleReferenceGroup(`namespace DefaultExportedFunction\n${fn}`, [r4]);
diff --git a/tests/cases/fourslash/findAllRefsForDefaultExport08.ts b/tests/cases/fourslash/findAllRefsForDefaultExport08.ts
index b29060f6099..5583ba34c95 100644
--- a/tests/cases/fourslash/findAllRefsForDefaultExport08.ts
+++ b/tests/cases/fourslash/findAllRefsForDefaultExport08.ts
@@ -10,6 +10,8 @@
 ////namespace [|{| "isWriteAccess": true, "isDefinition": true |}DefaultExportedClass|] {
 ////}
 
+verify.noErrors();
+
 // The namespace and class do not merge,
 // so the namespace should be all alone.
-verify.singleReferenceGroup("namespace DefaultExportedClass");
+verify.singleReferenceGroup("class DefaultExportedClass\nnamespace DefaultExportedClass");
diff --git a/tests/cases/fourslash/formattingOnEnter.ts b/tests/cases/fourslash/formattingOnEnter.ts
index 1c0915e839e..0e7d5af1a6d 100644
--- a/tests/cases/fourslash/formattingOnEnter.ts
+++ b/tests/cases/fourslash/formattingOnEnter.ts
@@ -6,4 +6,8 @@
 
 goTo.marker();
 edit.insertLine("");
-verify.currentLineContentIs('class bar {');
+verify.currentFileContentIs(
+`class foo { }
+class bar {
+}
+// new line here`);
diff --git a/tests/cases/fourslash/fourslash.ts b/tests/cases/fourslash/fourslash.ts
index 767701a2af6..59cb881ab7b 100644
--- a/tests/cases/fourslash/fourslash.ts
+++ b/tests/cases/fourslash/fourslash.ts
@@ -116,6 +116,7 @@ declare namespace FourSlashInterface {
         ranges(): Range[];
         rangesByText(): ts.Map<Range[]>;
         markerByName(s: string): Marker;
+        symbolsInScope(range: Range): any[];
     }
     class goTo {
         marker(name?: string | Marker): void;
@@ -192,6 +193,7 @@ declare namespace FourSlashInterface {
         verifyGetEmitOutputContentsForCurrentFile(expected: ts.OutputFile[]): void;
         noReferences(markerNameOrRange?: string | Range): void;
         symbolAtLocation(startRange: Range, ...declarationRanges: Range[]): void;
+        typeOfSymbolAtLocation(range: Range, symbol: any, expected: string): void;
         /**
          * @deprecated, prefer 'referenceGroups'
          * Like `referencesAre`, but goes to `start` first.
diff --git a/tests/cases/fourslash/protoVarInContexutalObjectLiteral.ts b/tests/cases/fourslash/protoVarInContextualObjectLiteral.ts
similarity index 100%
rename from tests/cases/fourslash/protoVarInContexutalObjectLiteral.ts
rename to tests/cases/fourslash/protoVarInContextualObjectLiteral.ts
diff --git a/tests/cases/fourslash/quickInfoOnNarrowedTypeInModule.ts b/tests/cases/fourslash/quickInfoOnNarrowedTypeInModule.ts
index 25e6cd4c68e..54a80194f37 100644
--- a/tests/cases/fourslash/quickInfoOnNarrowedTypeInModule.ts
+++ b/tests/cases/fourslash/quickInfoOnNarrowedTypeInModule.ts
@@ -7,7 +7,7 @@
 ////    var num: number;
 ////    var str: string;
 ////    if (typeof /*1*/nonExportedStrOrNum === "number") {
-////        num = /*2*/nonExportedStrOrNum; 
+////        num = /*2*/nonExportedStrOrNum;
 ////    }
 ////    else {
 ////        str = /*3*/nonExportedStrOrNum.length;
@@ -40,15 +40,15 @@ verify.completionListContains("nonExportedStrOrNum", "var nonExportedStrOrNum: s
 
 goTo.marker('4');
 verify.quickInfoIs('var m.exportedStrOrNum: string | number');
-verify.completionListContains("exportedStrOrNum", "var m.exportedStrOrNum: string | number");
+verify.completionListContains("exportedStrOrNum", "var exportedStrOrNum: string | number");
 
 goTo.marker('5');
 verify.quickInfoIs('var m.exportedStrOrNum: number');
-verify.completionListContains("exportedStrOrNum", "var m.exportedStrOrNum: number");
+verify.completionListContains("exportedStrOrNum", "var exportedStrOrNum: number");
 
 goTo.marker('6');
 verify.quickInfoIs('var m.exportedStrOrNum: string');
-verify.completionListContains("exportedStrOrNum", "var m.exportedStrOrNum: string");
+verify.completionListContains("exportedStrOrNum", "var exportedStrOrNum: string");
 
 goTo.marker('7');
 verify.quickInfoIs('var m.exportedStrOrNum: string | number');
diff --git a/tests/cases/fourslash/typeOfSymbol_localSymbolOfExport.ts b/tests/cases/fourslash/typeOfSymbol_localSymbolOfExport.ts
new file mode 100644
index 00000000000..4d23babbd21
--- /dev/null
+++ b/tests/cases/fourslash/typeOfSymbol_localSymbolOfExport.ts
@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts'/>
+
+////export function f() {}
+////[|1|];
+
+const ranges = test.ranges();
+const symbolsInScope = test.symbolsInScope(ranges[0]);
+const f = symbolsInScope.find(s => s.name === "f");
+if (f === undefined) throw new Error("'f' not in scope");
+if (f.exportSymbol === undefined) throw new Error("Expected to get the local symbol");
+
+verify.typeOfSymbolAtLocation(ranges[0], f, "() => void");
diff --git a/tests/cases/fourslash/unusedImports13FS.ts b/tests/cases/fourslash/unusedImports13FS.ts
new file mode 100644
index 00000000000..4e19f4d7f1a
--- /dev/null
+++ b/tests/cases/fourslash/unusedImports13FS.ts
@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts' />
+
+// @noUnusedLocals: true
+// @Filename: file2.ts
+//// [| import A, { x } from './a'; |]
+//// console.log(A);
+
+// @Filename: file1.ts
+//// export default 10;
+//// export var x = 10;
+
+verify.rangeAfterCodeFix("import A from './a';");
\ No newline at end of file
diff --git a/tests/cases/fourslash/unusedImports14FS.ts b/tests/cases/fourslash/unusedImports14FS.ts
new file mode 100644
index 00000000000..75124802d35
--- /dev/null
+++ b/tests/cases/fourslash/unusedImports14FS.ts
@@ -0,0 +1,15 @@
+/// <reference path='fourslash.ts' />
+
+// @noUnusedLocals: true
+// @Filename: file2.ts
+//// [| import /* 1 */ A /* 2 */, /* 3 */ { /* 4 */ x /* 5 */ } /* 6 */ from './a'; |]
+//// console.log(A);
+
+// @Filename: file1.ts
+//// export default 10;
+//// export var x = 10;
+
+
+// It's ambiguous which token comment /* 6 */ applies to or whether it should be removed.
+// In the current implementation the comment is left behind, but this behavior isn't a requirement.
+verify.rangeAfterCodeFix("import /* 1 */ A /* 2 */ /* 6 */ from './a';");
\ No newline at end of file