diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 4f1210793bf..ac2b88fbe93 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -752,7 +752,7 @@ namespace ts {
         }
     }
 
-    export function parseCommandLine(commandLine: string[], readFile?: (path: string) => string): ParsedCommandLine {
+    export function parseCommandLine(commandLine: string[], readFile?: (path: string) => string | undefined): ParsedCommandLine {
         const options: CompilerOptions = {};
         const fileNames: string[] = [];
         const errors: Diagnostic[] = [];
@@ -878,15 +878,9 @@ namespace ts {
      * Read tsconfig.json file
      * @param fileName The path to the config file
      */
-    export function readConfigFile(fileName: string, readFile: (path: string) => string): { config?: any; error?: Diagnostic } {
-        let text = "";
-        try {
-            text = readFile(fileName);
-        }
-        catch (e) {
-            return { config: {}, error: createCompilerDiagnostic(Diagnostics.Cannot_read_file_0_Colon_1, fileName, e.message) };
-        }
-        return parseConfigFileTextToJson(fileName, text);
+    export function readConfigFile(fileName: string, readFile: (path: string) => string | undefined): { config?: any; error?: Diagnostic } {
+        const textOrDiagnostic = tryReadFile(fileName, readFile);
+        return typeof textOrDiagnostic === "string" ? parseConfigFileTextToJson(fileName, textOrDiagnostic) : { config: {}, error: textOrDiagnostic };
     }
 
     /**
@@ -906,15 +900,20 @@ namespace ts {
      * Read tsconfig.json file
      * @param fileName The path to the config file
      */
-    export function readJsonConfigFile(fileName: string, readFile: (path: string) => string): JsonSourceFile {
-        let text = "";
+    export function readJsonConfigFile(fileName: string, readFile: (path: string) => string | undefined): JsonSourceFile {
+        const textOrDiagnostic = tryReadFile(fileName, readFile);
+        return typeof textOrDiagnostic === "string" ? parseJsonText(fileName, textOrDiagnostic) : <JsonSourceFile>{ parseDiagnostics: [textOrDiagnostic] };
+    }
+
+    function tryReadFile(fileName: string, readFile: (path: string) => string | undefined): string | Diagnostic {
+        let text: string | undefined;
         try {
             text = readFile(fileName);
         }
         catch (e) {
-            return <JsonSourceFile>{ parseDiagnostics: [createCompilerDiagnostic(Diagnostics.Cannot_read_file_0_Colon_1, fileName, e.message)] };
+            return createCompilerDiagnostic(Diagnostics.Cannot_read_file_0_Colon_1, fileName, e.message);
         }
-        return parseJsonText(fileName, text);
+        return text === undefined ? createCompilerDiagnostic(Diagnostics.The_specified_path_does_not_exist_Colon_0, fileName) : text;
     }
 
     function commandLineOptionsToMap(options: CommandLineOption[]) {
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 3e3ff90c00c..62ce272e96c 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -23,7 +23,7 @@ namespace ts {
         newLine: string;
         useCaseSensitiveFileNames: boolean;
         write(s: string): void;
-        readFile(path: string, encoding?: string): string;
+        readFile(path: string, encoding?: string): string | undefined;
         getFileSize?(path: string): number;
         writeFile(path: string, data: string, writeByteOrderMark?: boolean): void;
         /**
@@ -97,7 +97,7 @@ namespace ts {
         directoryExists(path: string): boolean;
         createDirectory(path: string): void;
         resolvePath(path: string): string;
-        readFile(path: string): string;
+        readFile(path: string): string | undefined;
         writeFile(path: string, contents: string): void;
         getDirectories(path: string): string[];
         readDirectory(path: string, extensions?: ReadonlyArray<string>, basePaths?: ReadonlyArray<string>, excludeEx?: string, includeFileEx?: string, includeDirEx?: string): string[];
@@ -204,7 +204,7 @@ namespace ts {
             const platform: string = _os.platform();
             const useCaseSensitiveFileNames = isFileSystemCaseSensitive();
 
-            function readFile(fileName: string, _encoding?: string): string {
+            function readFile(fileName: string, _encoding?: string): string | undefined {
                 if (!fileExists(fileName)) {
                     return undefined;
                 }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 6ba1591d54f..e252f9c3207 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2349,7 +2349,7 @@ namespace ts {
          */
         fileExists(path: string): boolean;
 
-        readFile(path: string): string;
+        readFile(path: string): string | undefined;
     }
 
     export interface WriteFileCallback {
@@ -3856,7 +3856,7 @@ namespace ts {
         fileExists(fileName: string): boolean;
         // readFile function is used to read arbitrary text files on disk, i.e. when resolution procedure needs the content of 'package.json'
         // to determine location of bundled typings for node module
-        readFile(fileName: string): string;
+        readFile(fileName: string): string | undefined;
         trace?(s: string): void;
         directoryExists?(directoryName: string): boolean;
         realpath?(path: string): string;
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 591baaab706..b7fabdda381 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -3925,7 +3925,7 @@ namespace ts {
      */
     export function validateLocaleAndSetLanguage(
         locale: string,
-        sys: { getExecutingFilePath(): string, resolvePath(path: string): string, fileExists(fileName: string): boolean, readFile(fileName: string): string },
+        sys: { getExecutingFilePath(): string, resolvePath(path: string): string, fileExists(fileName: string): boolean, readFile(fileName: string): string | undefined },
         errors?: Diagnostic[]) {
         const matchResult = /^([a-z]+)([_\-]([a-z]+))?$/.exec(locale.toLowerCase());
 
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index d5218695f09..55a8f3ebb4d 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -479,7 +479,7 @@ namespace Harness {
         getCurrentDirectory(): string;
         useCaseSensitiveFileNames(): boolean;
         resolvePath(path: string): string;
-        readFile(path: string): string;
+        readFile(path: string): string | undefined;
         writeFile(path: string, contents: string): void;
         directoryName(path: string): string;
         getDirectories(path: string): string[];
@@ -719,7 +719,7 @@ namespace Harness {
                 }
             });
 
-            export function readFile(file: string) {
+            export function readFile(file: string): string | undefined {
                 const response = Http.getFileFromServerSync(serverRoot + file);
                 if (response.status === 200) {
                     return response.responseText;
@@ -976,7 +976,7 @@ namespace Harness {
                 useCaseSensitiveFileNames: () => useCaseSensitiveFileNames,
                 getNewLine: () => newLine,
                 fileExists: fileName => fileMap.has(toPath(fileName)),
-                readFile: (fileName: string): string => {
+                readFile(fileName: string): string | undefined {
                     const file = fileMap.get(toPath(fileName));
                     if (ts.endsWith(fileName, "json")) {
                         // strip comments
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index 339138ba30f..156a9e08dce 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -215,7 +215,7 @@ namespace Harness.LanguageService {
                 depth,
                 (p) => this.virtualFileSystem.getAccessibleFileSystemEntries(p));
         }
-        readFile(path: string): string {
+        readFile(path: string): string | undefined {
             const snapshot = this.getScriptSnapshot(path);
             return snapshot.getText(0, snapshot.getLength());
         }
@@ -619,7 +619,7 @@ namespace Harness.LanguageService {
             this.writeMessage(message);
         }
 
-        readFile(fileName: string): string {
+        readFile(fileName: string): string | undefined {
             if (fileName.indexOf(Harness.Compiler.defaultLibFileName) >= 0) {
                 fileName = Harness.Compiler.defaultLibFileName;
             }
diff --git a/src/harness/projectsRunner.ts b/src/harness/projectsRunner.ts
index 46c2b42f6a0..7344c432b97 100644
--- a/src/harness/projectsRunner.ts
+++ b/src/harness/projectsRunner.ts
@@ -289,7 +289,7 @@ class ProjectRunner extends RunnerBase {
                 return Harness.IO.fileExists(getFileNameInTheProjectTest(fileName));
             }
 
-            function readFile(fileName: string): string {
+            function readFile(fileName: string): string | undefined {
                 return Harness.IO.readFile(getFileNameInTheProjectTest(fileName));
             }
 
diff --git a/src/harness/unittests/moduleResolution.ts b/src/harness/unittests/moduleResolution.ts
index 3923e4b2755..ffae1b5f4a9 100644
--- a/src/harness/unittests/moduleResolution.ts
+++ b/src/harness/unittests/moduleResolution.ts
@@ -56,7 +56,7 @@ namespace ts {
         else {
             return { readFile, fileExists: path => map.has(path) };
         }
-        function readFile(path: string): string {
+        function readFile(path: string): string | undefined {
             const file = map.get(path);
             return file && file.content;
         }
diff --git a/src/harness/unittests/session.ts b/src/harness/unittests/session.ts
index c6ec2042fae..862ebee4b03 100644
--- a/src/harness/unittests/session.ts
+++ b/src/harness/unittests/session.ts
@@ -9,7 +9,7 @@ namespace ts.server {
         newLine: "\n",
         useCaseSensitiveFileNames: true,
         write(s): void { lastWrittenToHost = s; },
-        readFile(): string { return void 0; },
+        readFile: () => undefined,
         writeFile: noop,
         resolvePath(): string { return void 0; },
         fileExists: () => false,
diff --git a/src/harness/virtualFileSystem.ts b/src/harness/virtualFileSystem.ts
index da7784770df..8accd6f621e 100644
--- a/src/harness/virtualFileSystem.ts
+++ b/src/harness/virtualFileSystem.ts
@@ -209,7 +209,7 @@ namespace Utils {
             }
         }
 
-        readFile(path: string): string {
+        readFile(path: string): string | undefined {
             const value = this.traversePath(path);
             if (value && value.isFile()) {
                 return value.content.content;
diff --git a/src/server/lsHost.ts b/src/server/lsHost.ts
index 3f1a4e054a8..13b9505a658 100644
--- a/src/server/lsHost.ts
+++ b/src/server/lsHost.ts
@@ -217,7 +217,7 @@ namespace ts.server {
             return !this.project.isWatchedMissingFile(path) && this.host.fileExists(file);
         }
 
-        readFile(fileName: string): string {
+        readFile(fileName: string): string | undefined {
             return this.host.readFile(fileName);
         }
 
diff --git a/src/server/typingsInstaller/typingsInstaller.ts b/src/server/typingsInstaller/typingsInstaller.ts
index 730257d2ccc..b1fddfb4129 100644
--- a/src/server/typingsInstaller/typingsInstaller.ts
+++ b/src/server/typingsInstaller/typingsInstaller.ts
@@ -93,10 +93,10 @@ namespace ts.server.typingsInstaller {
         abstract readonly typesRegistry: Map<void>;
 
         constructor(
-            readonly installTypingHost: InstallTypingHost,
-            readonly globalCachePath: string,
-            readonly safeListPath: Path,
-            readonly throttleLimit: number,
+            protected readonly installTypingHost: InstallTypingHost,
+            private readonly globalCachePath: string,
+            private readonly safeListPath: Path,
+            private readonly throttleLimit: number,
             protected readonly log = nullLog) {
             if (this.log.isEnabled()) {
                 this.log.writeLine(`Global cache location '${globalCachePath}', safe file path '${safeListPath}'`);
diff --git a/src/services/jsTyping.ts b/src/services/jsTyping.ts
index f31276b6498..670633eb44b 100644
--- a/src/services/jsTyping.ts
+++ b/src/services/jsTyping.ts
@@ -9,10 +9,10 @@
 namespace ts.JsTyping {
 
     export interface TypingResolutionHost {
-        directoryExists: (path: string) => boolean;
-        fileExists: (fileName: string) => boolean;
-        readFile: (path: string, encoding?: string) => string;
-        readDirectory: (rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, depth?: number) => string[];
+        directoryExists(path: string): boolean;
+        fileExists(fileName: string): boolean;
+        readFile(path: string, encoding?: string): string | undefined;
+        readDirectory(rootDir: string, extensions: ReadonlyArray<string>, excludes: ReadonlyArray<string>, includes: ReadonlyArray<string>, depth?: number): string[];
     }
 
     interface PackageJson {
diff --git a/src/services/services.ts b/src/services/services.ts
index 5b2cf949d2a..30c6b88b1e2 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1149,7 +1149,7 @@ namespace ts {
                         !!hostCache.getEntryByPath(path) :
                         (host.fileExists && host.fileExists(fileName));
                 },
-                readFile: (fileName): string => {
+                readFile(fileName) {
                     // stub missing host functionality
                     const path = toPath(fileName, currentDirectory, getCanonicalFileName);
                     if (hostCache.containsEntryByPath(path)) {
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 2ee1853f18f..03965cb5d48 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -69,7 +69,7 @@ namespace ts {
 
         getTypeRootsVersion?(): number;
         readDirectory(rootDir: string, extension: string, basePaths?: string, excludeEx?: string, includeFileEx?: string, includeDirEx?: string, depth?: number): string;
-        readFile(path: string, encoding?: string): string;
+        readFile(path: string, encoding?: string): string | undefined;
         fileExists(path: string): boolean;
 
         getModuleResolutionsForFile?(fileName: string): string;
@@ -95,7 +95,7 @@ namespace ts {
         /**
          * Read arbitary text files on disk, i.e. when resolution procedure needs the content of 'package.json' to determine location of bundled typings for node modules
          */
-        readFile(fileName: string): string;
+        readFile(fileName: string): string | undefined;
         realpath?(path: string): string;
         trace(s: string): void;
         useCaseSensitiveFileNames?(): boolean;
@@ -458,7 +458,7 @@ namespace ts {
             ));
         }
 
-        public readFile(path: string, encoding?: string): string {
+        public readFile(path: string, encoding?: string): string | undefined {
             return this.shimHost.readFile(path, encoding);
         }
 
@@ -501,7 +501,7 @@ namespace ts {
             return this.shimHost.fileExists(fileName);
         }
 
-        public readFile(fileName: string): string {
+        public readFile(fileName: string): string | undefined {
             return this.shimHost.readFile(fileName);
         }
 
@@ -1006,7 +1006,7 @@ namespace ts {
     class CoreServicesShimObject extends ShimBase implements CoreServicesShim {
         private logPerformance = false;
 
-        constructor(factory: ShimFactory, public logger: Logger, private host: CoreServicesShimHostAdapter) {
+        constructor(factory: ShimFactory, public readonly logger: Logger, private readonly host: CoreServicesShimHostAdapter) {
             super(factory);
         }
 
diff --git a/src/services/types.ts b/src/services/types.ts
index bfb8a80b769..19d2345d968 100644
--- a/src/services/types.ts
+++ b/src/services/types.ts
@@ -165,7 +165,7 @@ namespace ts {
          * Without these methods, only completions for ambient modules will be provided.
          */
         readDirectory?(path: string, extensions?: ReadonlyArray<string>, exclude?: ReadonlyArray<string>, include?: ReadonlyArray<string>, depth?: number): string[];
-        readFile?(path: string, encoding?: string): string;
+        readFile?(path: string, encoding?: string): string | undefined;
         fileExists?(path: string): boolean;
 
         /*