diff --git a/Gulpfile.ts b/Gulpfile.ts
index 6c91ef52cbb49..2a0e7092931e5 100644
--- a/Gulpfile.ts
+++ b/Gulpfile.ts
@@ -411,7 +411,7 @@ gulp.task(servicesFile, false, ["lib", "generate-diagnostics"], () => {
         completedDts.pipe(clone())
             .pipe(insert.transform((content, file) => {
                 file.path = nodeStandaloneDefinitionsFile;
-                return content.replace(/declare (namespace|module) ts/g, 'declare module "typescript"');
+                return content.replace(/declare (namespace|module) ts {/g, 'declare module "typescript" {\n    import * as ts from "typescript";');
             }))
     ]).pipe(gulp.dest(builtLocalDirectory));
 });
diff --git a/Jakefile.js b/Jakefile.js
index 174be5e702f52..868c3cd53e2ae 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -46,6 +46,7 @@ var compilerSources = [
     "declarationEmitter.ts",
     "emitter.ts",
     "program.ts",
+    "extensions.ts",
     "commandLineParser.ts",
     "tsc.ts",
     "diagnosticInformationMap.generated.ts"
@@ -67,6 +68,7 @@ var servicesSources = [
     "declarationEmitter.ts",
     "emitter.ts",
     "program.ts",
+    "extensions.ts",
     "commandLineParser.ts",
     "diagnosticInformationMap.generated.ts"
 ].map(function (f) {
@@ -131,6 +133,7 @@ var harnessCoreSources = [
     "typeWriter.ts",
     "fourslashRunner.ts",
     "projectsRunner.ts",
+    "extensionRunner.ts",
     "loggedIO.ts",
     "rwcRunner.ts",
     "test262Runner.ts",
@@ -158,7 +161,7 @@ var harnessSources = harnessCoreSources.concat([
     "convertCompilerOptionsFromJson.ts",
     "convertTypingOptionsFromJson.ts",
     "tsserverProjectSystem.ts",
-    "matchFiles.ts"
+    "matchFiles.ts",
 ].map(function (f) {
     return path.join(unittestsDirectory, f);
 })).concat([
@@ -524,7 +527,7 @@ compileFile(servicesFile, servicesSources,[builtLocalDirectory, copyright].conca
 
                 // Node package definition file to be distributed without the package. Created by replacing
                 // 'ts' namespace with '"typescript"' as a module.
-                var nodeStandaloneDefinitionsFileContents = definitionFileContents.replace(/declare (namespace|module) ts/g, 'declare module "typescript"');
+                var nodeStandaloneDefinitionsFileContents = definitionFileContents.replace(/declare (namespace|module) ts {/g, 'declare module "typescript" {\n    import * as ts from "typescript";');
                 fs.writeFileSync(nodeStandaloneDefinitionsFile, nodeStandaloneDefinitionsFileContents);
             });
 
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index 7e2c6eb8d334e..8dd35b033f031 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -282,6 +282,12 @@ namespace ts {
             experimental: true,
             description: Diagnostics.Enables_experimental_support_for_emitting_type_metadata_for_decorators
         },
+        {
+            name: "extensions",
+            type: "object",
+            isTSConfigOnly: true,
+            description: Diagnostics.List_of_compiler_extensions_to_require
+        },
         {
             name: "moduleResolution",
             type: {
@@ -429,7 +435,7 @@ namespace ts {
             name: "strictNullChecks",
             type: "boolean",
             description: Diagnostics.Enable_strict_null_checks
-        }
+        },
     ];
 
     /* @internal */
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 6c87ad82955c3..f12f110e3e39f 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -2,6 +2,17 @@
 /// <reference path="performance.ts" />
 
 
+namespace ts {
+    export function startsWith(str: string, prefix: string): boolean {
+        return str.lastIndexOf(prefix, 0) === 0;
+    }
+
+    export function endsWith(str: string, suffix: string): boolean {
+        const expectedPos = str.length - suffix.length;
+        return expectedPos >= 0 && str.indexOf(suffix, expectedPos) === expectedPos;
+    }
+}
+
 /* @internal */
 namespace ts {
     /**
@@ -178,6 +189,26 @@ namespace ts {
         return array1.concat(array2);
     }
 
+    export function flatten<T>(array1: T[][]): T[] {
+        if (!array1 || !array1.length) return <any>array1;
+        return [].concat(...array1);
+    }
+
+    export function groupBy<T>(array: T[], classifier: (item: T) => string): {[index: string]: T[]};
+    export function groupBy<T>(array: T[], classifier: (item: T) => number): {[index: number]: T[]};
+    export function groupBy<T>(array: T[], classifier: (item: T) => (string | number)): {[index: string]: T[], [index: number]: T[]} {
+        if (!array || !array.length) return undefined;
+        const ret: {[index: string]: T[], [index: number]: T[]} = {};
+        for (const elem of array) {
+            const key = classifier(elem);
+            if (!ret[key]) {
+                ret[key] = [];
+            }
+            ret[key].push(elem);
+        }
+        return ret;
+    }
+
     export function deduplicate<T>(array: T[], areEqual?: (a: T, b: T) => boolean): T[] {
         let result: T[];
         if (array) {
@@ -908,17 +939,6 @@ namespace ts {
         return true;
     }
 
-    /* @internal */
-    export function startsWith(str: string, prefix: string): boolean {
-        return str.lastIndexOf(prefix, 0) === 0;
-    }
-
-    /* @internal */
-    export function endsWith(str: string, suffix: string): boolean {
-        const expectedPos = str.length - suffix.length;
-        return expectedPos >= 0 && str.indexOf(suffix, expectedPos) === expectedPos;
-    }
-
     export function fileExtensionIs(path: string, extension: string): boolean {
         return path.length > extension.length && endsWith(path, extension);
     }
@@ -1195,7 +1215,8 @@ namespace ts {
     export const supportedJavascriptExtensions = [".js", ".jsx"];
     const allSupportedExtensions  = supportedTypeScriptExtensions.concat(supportedJavascriptExtensions);
 
-    export function getSupportedExtensions(options?: CompilerOptions): string[] {
+    export function getSupportedExtensions(options?: CompilerOptions, loadJS?: boolean): string[] {
+        if (loadJS) return supportedJavascriptExtensions;
         return options && options.allowJs ? allSupportedExtensions : supportedTypeScriptExtensions;
     }
 
@@ -1373,4 +1394,49 @@ namespace ts {
             : ((fileName) => fileName.toLowerCase());
     }
 
+    /**
+     * This isn't the strictest deep equal, but it's good enough for us
+     *  - +0 === -0 (though who really wants to consider them different?)
+     *  - arguments and arrays can be equal (both typeof === object, both have enumerable keys)
+     *  - doesn't inspect es6 iterables (not that they're used in this code base)
+     *  - doesn't inspect regex toString value (so only references to the same regex are equal)
+     *  - doesn't inspect date primitive number value (so only references to the same date are equal)
+     */
+    export function deepEqual(a: any, b: any, memo?: [any, any][]): boolean {
+        if (a === b) return true;
+        if (typeof a !== typeof b) return false;
+        // Special case NaN
+        if (typeof a === "number" && isNaN(a) && isNaN(b)) return true;
+        // We can't know if function arguments are deep equal, so we say they're equal if they look alike
+        if (typeof a === "object" || typeof a === "function") {
+            if (memo) {
+                for (let i = 0; i < memo.length; i++) {
+                    if (memo[i][0] === a && memo[i][1] === b) return true;
+                    if (memo[i][0] === b && memo[i][1] === a) return true;
+                }
+            }
+            else {
+                memo = [];
+            }
+
+            const aKeys = ts.getKeys(a);
+            const bKeys = ts.getKeys(b);
+            aKeys.sort();
+            bKeys.sort();
+
+            if (aKeys.length !== bKeys.length) return false;
+
+            for (let i = 0; i < aKeys.length; i++) {
+                if (aKeys[i] !== bKeys[i]) return false;
+            }
+
+            memo.push([a, b]);
+
+            for (const key of aKeys) {
+                if (!deepEqual(a[key], b[key], memo)) return false;
+            }
+            return true;
+        }
+        return false;
+    }
 }
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 8126d5c605ea7..2a73331b91b0e 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2676,7 +2676,7 @@
         "category": "Message",
         "code": 6099
     },
-    "'package.json' does not have 'types' field.": {
+    "'package.json' does not have '{0}' field.": {
         "category": "Message",
         "code": 6100
     },
@@ -2696,7 +2696,7 @@
         "category": "Message",
         "code": 6104
     },
-    "Expected type of '{0}' field in 'package.json' to be 'string', got '{1}'.": {
+    "Expected type of '{0}' field in 'package.json' to be '{1}', got '{2}'.": {
         "category": "Message",
         "code": 6105
     },
@@ -2824,10 +2824,20 @@
         "category": "Message",
         "code": 6136
     },
-    "No types specified in 'package.json' but 'allowJs' is set, so returning 'main' value of '{0}'": {
+
+    "List of compiler extensions to require.": {
         "category": "Message",
-        "code": 6137
+        "code": 6150
+    },
+    "Extension loading failed with error '{0}'.": {
+        "category": "Error",
+        "code": 6151
     },
+    "Extension '{0}' exported member '{1}' has extension kind '{2}', but was type '{3}' when type '{4}' was expected.": {
+        "category": "Error",
+        "code": 6152
+    },
+
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
diff --git a/src/compiler/extensions.ts b/src/compiler/extensions.ts
new file mode 100644
index 0000000000000..dd78702b67f85
--- /dev/null
+++ b/src/compiler/extensions.ts
@@ -0,0 +1,226 @@
+namespace ts {
+
+    export interface BaseProviderStatic {
+        readonly ["extension-kind"]: ExtensionKind;
+        new (state: {ts: typeof ts, args: any}): any;
+    }
+
+    export interface LanguageServiceHost {} // The members for these interfaces are provided in the services layer
+    export interface LanguageService {}
+    export interface LanguageServiceProvider {}
+    export interface DocumentRegistry {}
+
+    export interface LanguageServiceProviderStatic extends BaseProviderStatic {
+        readonly ["extension-kind"]: ExtensionKind.LanguageService;
+        new (state: { ts: typeof ts, args: any, host: LanguageServiceHost, service: LanguageService, registry: DocumentRegistry }): LanguageServiceProvider;
+    }
+
+    export namespace ExtensionKind {
+        export const LanguageService: "language-service" = "language-service";
+        export type LanguageService = "language-service";
+    }
+    export type ExtensionKind = ExtensionKind.LanguageService;
+
+    export interface ExtensionCollectionMap {
+        "language-service"?: LanguageServiceExtension[];
+        [index: string]: Extension[] | undefined;
+    }
+
+    export interface ExtensionBase {
+        name: string;
+        args: any;
+        kind: ExtensionKind;
+    }
+
+    export interface ProfileData {
+        globalBucket: string;
+        task: string;
+        start: number;
+        length?: number;
+    }
+
+    export interface LanguageServiceExtension extends ExtensionBase {
+        kind: ExtensionKind.LanguageService;
+        ctor: LanguageServiceProviderStatic;
+    }
+
+    export type Extension = LanguageServiceExtension;
+
+    export interface ExtensionCache {
+        getCompilerExtensions(): ExtensionCollectionMap;
+        getExtensionLoadingDiagnostics(): Diagnostic[];
+    }
+
+    export interface ExtensionHost extends ModuleResolutionHost {
+        loadExtension?(name: string): any;
+    }
+
+    export interface Program {
+        /**
+         * Gets a map of loaded compiler extensions
+         */
+        getCompilerExtensions(): ExtensionCollectionMap;
+
+        /**
+         * Gets only diagnostics reported while loading extensions
+         */
+        getExtensionLoadingDiagnostics(): Diagnostic[];
+    }
+
+    /* @internal */
+    export interface TypeCheckerHost {
+        getCompilerExtensions(): ExtensionCollectionMap;
+    }
+
+    export const perfTraces: Map<ProfileData> = {};
+
+    function getExtensionRootName(qualifiedName: string) {
+        return qualifiedName.substring(0, qualifiedName.indexOf("[")) || qualifiedName;
+    }
+
+    function createTaskName(qualifiedName: string, task: string) {
+        return `${task}|${qualifiedName}`;
+    }
+
+    export function startProfile(enabled: boolean, key: string, bucket?: string) {
+        if (!enabled) return;
+        performance.emit(`start|${key}`);
+        perfTraces[key] = {
+            task: key,
+            start: performance.mark(),
+            length: undefined,
+            globalBucket: bucket
+        };
+    }
+
+    export function completeProfile(enabled: boolean, key: string) {
+        if (!enabled) return;
+        Debug.assert(!!perfTraces[key], "Completed profile did not have a corresponding start.");
+        perfTraces[key].length = performance.measure(perfTraces[key].globalBucket, perfTraces[key].start);
+        performance.emit(`end|${key}`);
+    }
+
+    export function startExtensionProfile(enabled: boolean, qualifiedName: string, task: string) {
+        if (!enabled) return;
+        const longTask = createTaskName(qualifiedName, task);
+        startProfile(/*enabled*/true, longTask, getExtensionRootName(qualifiedName));
+    }
+
+    export function completeExtensionProfile(enabled: boolean, qualifiedName: string, task: string) {
+        if (!enabled) return;
+        const longTask = createTaskName(qualifiedName, task);
+        completeProfile(/*enabled*/true, longTask);
+    }
+
+    export function createExtensionCache(options: CompilerOptions, host: ExtensionHost, resolvedExtensionNames?: Map<string>): ExtensionCache {
+
+        const diagnostics: Diagnostic[] = [];
+        const extOptions = options.extensions;
+        const extensionNames = (extOptions instanceof Array) ? extOptions : getKeys(extOptions);
+        // Eagerly evaluate extension paths, but lazily execute their contents
+        resolvedExtensionNames = resolvedExtensionNames || resolveExtensionNames();
+        let extensions: ExtensionCollectionMap;
+
+        const cache: ExtensionCache = {
+            getCompilerExtensions: () => {
+                if (!extensions) {
+                    extensions = collectCompilerExtensions();
+                }
+                return extensions;
+            },
+            getExtensionLoadingDiagnostics: () => {
+                // To get extension loading diagnostics, we need to make sure we've actually loaded them
+                cache.getCompilerExtensions();
+                return diagnostics;
+            },
+        };
+        return cache;
+
+        function resolveExtensionNames(): Map<string> {
+            const currentDirectory = host.getCurrentDirectory ? host.getCurrentDirectory() : "";
+            const extMap: Map<string> = {};
+            forEach(extensionNames, name => {
+                const resolved = resolveModuleName(name, combinePaths(currentDirectory, "tsconfig.json"), options, host, /*loadJs*/true).resolvedModule;
+                if (resolved) {
+                    extMap[name] = resolved.resolvedFileName;
+                }
+            });
+            return extMap;
+        }
+
+        function collectCompilerExtensions(): ExtensionCollectionMap {
+            const profilingEnabled = options.extendedDiagnostics;
+            const extensionLoadResults = map(extensionNames, (name) => {
+                const resolved = resolvedExtensionNames[name];
+                let result: any;
+                let error: any;
+                if (!resolved) {
+                    error = new Error(`Host could not locate extension '${name}'.`);
+                }
+                if (resolved && host.loadExtension) {
+                    try {
+                        startProfile(profilingEnabled, name, name);
+                        result = host.loadExtension(resolved);
+                        completeProfile(profilingEnabled, name);
+                    }
+                    catch (e) {
+                        error = e;
+                    }
+                }
+                else if (!host.loadExtension) {
+                    error = new Error("Extension loading not implemented in host!");
+                }
+                if (error) {
+                    diagnostics.push(createCompilerDiagnostic(Diagnostics.Extension_loading_failed_with_error_0, error));
+                }
+                return { name, result, error };
+            });
+            const successfulExtensionLoadResults = filter(extensionLoadResults, res => !res.error);
+            const preparedExtensionObjects = map(successfulExtensionLoadResults, res => {
+                if (!res.result) {
+                    return [];
+                }
+                const aggregate: Extension[] = [];
+                forEachKey(res.result, key => {
+                    const potentialExtension = res.result[key];
+                    if (!potentialExtension) {
+                        return; // Avoid errors on explicitly exported null/undefined (why would someone do that, though?)
+                    }
+                    const annotatedKind = potentialExtension["extension-kind"];
+                    if (typeof annotatedKind !== "string") {
+                        return;
+                    }
+                    const ext: ExtensionBase = {
+                        name: key !== "default" ? `${res.name}[${key}]` : res.name,
+                        args: extensionNames === extOptions ? undefined : (extOptions as Map<any>)[res.name],
+                        kind: annotatedKind as ExtensionKind,
+                    };
+                    switch (ext.kind) {
+                        case ExtensionKind.LanguageService:
+                            if (typeof potentialExtension !== "function") {
+                                diagnostics.push(createCompilerDiagnostic(
+                                    Diagnostics.Extension_0_exported_member_1_has_extension_kind_2_but_was_type_3_when_type_4_was_expected,
+                                    res.name,
+                                    key,
+                                    (ts as any).ExtensionKind[annotatedKind],
+                                    typeof potentialExtension,
+                                    "function"
+                                ));
+                                return;
+                            }
+                            (ext as LanguageServiceExtension).ctor = potentialExtension as LanguageServiceProviderStatic;
+                            break;
+                        default:
+                            // Include a default case which just puts the extension unchecked onto the base extension
+                            // This can allow language service extensions to query for custom extension kinds
+                            (ext as any).__extension = potentialExtension;
+                            break;
+                    }
+                    aggregate.push(ext as Extension);
+                });
+                return aggregate;
+            });
+            return groupBy(flatten(preparedExtensionObjects), elem => elem.kind) || {};
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/compiler/performance.ts b/src/compiler/performance.ts
index 89db876ae5e48..71f3dbd74854d 100644
--- a/src/compiler/performance.ts
+++ b/src/compiler/performance.ts
@@ -58,10 +58,11 @@ namespace ts.performance {
      * @param measureName The name of the performance measurement.
      * @param marker The timestamp of the starting mark.
      */
-    export function measure(measureName: string, marker: number) {
+    export function measure(measureName: string, marker: number): number {
         if (measures) {
-            measures[measureName] = (getProperty(measures, measureName) || 0) + (timestamp() - marker);
+            return measures[measureName] = (getProperty(measures, measureName) || 0) + (timestamp() - marker);
         }
+        return 0;
     }
 
     /**
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 7d40b2f3219fc..9ba91f41bac81 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -1,6 +1,8 @@
 /// <reference path="sys.ts" />
 /// <reference path="emitter.ts" />
 /// <reference path="core.ts" />
+/// <reference path="extensions.ts" />
+
 
 namespace ts {
     /** The version of the TypeScript compiler release */
@@ -118,57 +120,51 @@ namespace ts {
         skipTsx: boolean;
     }
 
-    function tryReadTypesSection(packageJsonPath: string, baseDirectory: string, state: ModuleResolutionState): string {
-        let jsonContent: { typings?: string, types?: string, main?: string };
-        try {
-            const jsonText = state.host.readFile(packageJsonPath);
-            jsonContent = jsonText ? <{ typings?: string, types?: string, main?: string }>JSON.parse(jsonText) : {};
+    function getPackageEntry(packageJson: any, key: string, tag: string, state: ModuleResolutionState) {
+        const value = packageJson[key];
+        if (typeof value === tag) {
+            return value;
         }
-        catch (e) {
-            // gracefully handle if readFile fails or returns not JSON
-            jsonContent = {};
+        if (state.traceEnabled) {
+            trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_1_got_2, key, tag, typeof value);
         }
+        return undefined;
+    }
 
-        let typesFile: string;
-        let fieldName: string;
-        // first try to read content of 'typings' section (backward compatibility)
-        if (jsonContent.typings) {
-            if (typeof jsonContent.typings === "string") {
-                fieldName = "typings";
-                typesFile = jsonContent.typings;
-            }
-            else {
-                if (state.traceEnabled) {
-                    trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_string_got_1, "typings", typeof jsonContent.typings);
-                }
-            }
+    function getPackageEntryAsPath(packageJson: any, packageJsonPath: string, key: string, state: ModuleResolutionState) {
+        const value = getPackageEntry(packageJson, key, "string", state);
+        const path = value ? normalizePath(combinePaths(getDirectoryPath(packageJsonPath), value)) : undefined;
+        if (path && state.traceEnabled) {
+            trace(state.host, Diagnostics.package_json_has_0_field_1_that_references_2, key, value, path);
         }
-        // then read 'types'
-        if (!typesFile && jsonContent.types) {
-            if (typeof jsonContent.types === "string") {
-                fieldName = "types";
-                typesFile = jsonContent.types;
-            }
-            else {
-                if (state.traceEnabled) {
-                    trace(state.host, Diagnostics.Expected_type_of_0_field_in_package_json_to_be_string_got_1, "types", typeof jsonContent.types);
-                }
-            }
+        return path;
+    }
+
+    function getPackageTypes(packageJsonPath: string, state: ModuleResolutionState) {
+        const { config } = readConfigFile(packageJsonPath, state.host.readFile);
+        if (config) {
+            return getPackageEntryAsPath(config, packageJsonPath, "typings", state)
+                || getPackageEntryAsPath(config, packageJsonPath, "types", state)
+                // Use the main module for inferring types if no types package specified and the allowJs is set
+                || (state.compilerOptions.allowJs && getPackageEntryAsPath(config, packageJsonPath, "main", state));
         }
-        if (typesFile) {
-            const typesFilePath = normalizePath(combinePaths(baseDirectory, typesFile));
+        else {
             if (state.traceEnabled) {
-                trace(state.host, Diagnostics.package_json_has_0_field_1_that_references_2, fieldName, typesFile, typesFilePath);
+                trace(state.host, Diagnostics.package_json_does_not_have_0_field, "types");
             }
-            return typesFilePath;
         }
-        // Use the main module for inferring types if no types package specified and the allowJs is set
-        if (state.compilerOptions.allowJs && jsonContent.main && typeof jsonContent.main === "string") {
+        return undefined;
+    }
+
+    function getPackageMain(packageJsonPath: string, state: ModuleResolutionState) {
+        const { config } = readConfigFile(packageJsonPath, state.host.readFile);
+        if (config) {
+            return getPackageEntryAsPath(config, packageJsonPath, "main", state);
+        }
+        else {
             if (state.traceEnabled) {
-                trace(state.host, Diagnostics.No_types_specified_in_package_json_but_allowJs_is_set_so_returning_main_value_of_0, jsonContent.main);
+                trace(state.host, Diagnostics.package_json_does_not_have_0_field, "main");
             }
-            const mainFilePath = normalizePath(combinePaths(baseDirectory, jsonContent.main));
-            return mainFilePath;
         }
         return undefined;
     }
@@ -293,7 +289,7 @@ namespace ts {
         };
     }
 
-    export function resolveModuleName(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost): ResolvedModuleWithFailedLookupLocations {
+    export function resolveModuleName(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, loadJs?: boolean): ResolvedModuleWithFailedLookupLocations {
         const traceEnabled = isTraceEnabled(compilerOptions, host);
         if (traceEnabled) {
             trace(host, Diagnostics.Resolving_module_0_from_1, moduleName, containingFile);
@@ -315,7 +311,7 @@ namespace ts {
         let result: ResolvedModuleWithFailedLookupLocations;
         switch (moduleResolution) {
             case ModuleResolutionKind.NodeJs:
-                result = nodeModuleNameResolver(moduleName, containingFile, compilerOptions, host);
+                result = nodeModuleNameResolver(moduleName, containingFile, compilerOptions, host, loadJs);
                 break;
             case ModuleResolutionKind.Classic:
                 result = classicNameResolver(moduleName, containingFile, compilerOptions, host);
@@ -610,7 +606,7 @@ namespace ts {
         };
     }
 
-    export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost): ResolvedModuleWithFailedLookupLocations {
+    export function nodeModuleNameResolver(moduleName: string, containingFile: string, compilerOptions: CompilerOptions, host: ModuleResolutionHost, loadJs?: boolean): ResolvedModuleWithFailedLookupLocations {
         const containingDirectory = getDirectoryPath(containingFile);
         const supportedExtensions = getSupportedExtensions(compilerOptions);
         const traceEnabled = isTraceEnabled(compilerOptions, host);
@@ -626,7 +622,7 @@ namespace ts {
                 if (traceEnabled) {
                     trace(host, Diagnostics.Loading_module_0_from_node_modules_folder, moduleName);
                 }
-                resolvedFileName = loadModuleFromNodeModules(moduleName, containingDirectory, failedLookupLocations, state);
+                resolvedFileName = loadModuleFromNodeModules(moduleName, containingDirectory, failedLookupLocations, state, loadJs);
                 isExternalLibraryImport = resolvedFileName !== undefined;
             }
             else {
@@ -716,25 +712,20 @@ namespace ts {
         }
     }
 
-    function loadNodeModuleFromDirectory(extensions: string[], candidate: string, failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState): string {
+    function loadNodeModuleFromDirectory(extensions: string[], candidate: string, failedLookupLocation: string[], onlyRecordFailures: boolean, state: ModuleResolutionState, loadJS?: boolean): string {
         const packageJsonPath = combinePaths(candidate, "package.json");
         const directoryExists = !onlyRecordFailures && directoryProbablyExists(candidate, state.host);
         if (directoryExists && state.host.fileExists(packageJsonPath)) {
             if (state.traceEnabled) {
                 trace(state.host, Diagnostics.Found_package_json_at_0, packageJsonPath);
             }
-            const typesFile = tryReadTypesSection(packageJsonPath, candidate, state);
+            const typesFile = loadJS ? getPackageMain(packageJsonPath, state) : getPackageTypes(packageJsonPath, state);
             if (typesFile) {
                 const result = loadModuleFromFile(typesFile, extensions, failedLookupLocation, !directoryProbablyExists(getDirectoryPath(typesFile), state.host), state);
                 if (result) {
                     return result;
                 }
             }
-            else {
-                if (state.traceEnabled) {
-                    trace(state.host, Diagnostics.package_json_does_not_have_types_field);
-                }
-            }
         }
         else {
             if (state.traceEnabled) {
@@ -747,30 +738,31 @@ namespace ts {
         return loadModuleFromFile(combinePaths(candidate, "index"), extensions, failedLookupLocation, !directoryExists, state);
     }
 
-    function loadModuleFromNodeModulesFolder(moduleName: string, directory: string, failedLookupLocations: string[], state: ModuleResolutionState): string {
+    function loadModuleFromNodeModulesFolder(moduleName: string, directory: string, failedLookupLocations: string[], state: ModuleResolutionState, loadJS?: boolean): string {
         const nodeModulesFolder = combinePaths(directory, "node_modules");
         const nodeModulesFolderExists = directoryProbablyExists(nodeModulesFolder, state.host);
         const candidate = normalizePath(combinePaths(nodeModulesFolder, moduleName));
-        const supportedExtensions = getSupportedExtensions(state.compilerOptions);
+
+        const supportedExtensions = getSupportedExtensions(state.compilerOptions, loadJS);
 
         let result = loadModuleFromFile(candidate, supportedExtensions, failedLookupLocations, !nodeModulesFolderExists, state);
         if (result) {
             return result;
         }
-        result = loadNodeModuleFromDirectory(supportedExtensions, candidate, failedLookupLocations, !nodeModulesFolderExists, state);
+        result = loadNodeModuleFromDirectory(supportedExtensions, candidate, failedLookupLocations, !nodeModulesFolderExists, state, loadJS);
         if (result) {
             return result;
         }
     }
 
-    function loadModuleFromNodeModules(moduleName: string, directory: string, failedLookupLocations: string[], state: ModuleResolutionState): string {
+    function loadModuleFromNodeModules(moduleName: string, directory: string, failedLookupLocations: string[], state: ModuleResolutionState, loadJS?: boolean): string {
         directory = normalizeSlashes(directory);
         while (true) {
             const baseName = getBaseFileName(directory);
             if (baseName !== "node_modules") {
                 // Try to load source from the package
-                const packageResult = loadModuleFromNodeModulesFolder(moduleName, directory, failedLookupLocations, state);
-                if (packageResult && hasTypeScriptFileExtension(packageResult)) {
+                const packageResult = loadModuleFromNodeModulesFolder(moduleName, directory, failedLookupLocations, state, loadJS);
+                if (packageResult && (hasTypeScriptFileExtension(packageResult) || loadJS)) {
                     // Always prefer a TypeScript (.ts, .tsx, .d.ts) file shipped with the package
                     return packageResult;
                 }
@@ -954,6 +946,7 @@ namespace ts {
 
         const newLine = getNewLineCharacter(options);
         const realpath = sys.realpath && ((path: string) => sys.realpath(path));
+        const loadExtension = sys.loadExtension && ((name: string) => sys.loadExtension(name));
 
         return {
             getSourceFile,
@@ -969,7 +962,8 @@ namespace ts {
             trace: (s: string) => sys.write(s + newLine),
             directoryExists: directoryName => sys.directoryExists(directoryName),
             getDirectories: (path: string) => sys.getDirectories(path),
-            realpath
+            realpath,
+            loadExtension
         };
     }
 
@@ -1004,7 +998,8 @@ namespace ts {
             }
 
             const category = DiagnosticCategory[diagnostic.category].toLowerCase();
-            output += `${ category } TS${ diagnostic.code }: ${ flattenDiagnosticMessageText(diagnostic.messageText, host.getNewLine()) }${ host.getNewLine() }`;
+            const code = typeof diagnostic.code === "string" ? diagnostic.code : `TS${ diagnostic.code }`;
+            output += `${ category } ${ code }: ${ flattenDiagnosticMessageText(diagnostic.messageText, host.getNewLine()) }${ host.getNewLine() }`;
         }
         return output;
     }
@@ -1087,7 +1082,7 @@ namespace ts {
         return result;
     }
 
-    export function createProgram(rootNames: string[], options: CompilerOptions, host?: CompilerHost, oldProgram?: Program): Program {
+    export function createProgram(rootNames: string[], options: CompilerOptions, host?: CompilerHost, oldProgram?: Program, extensionCache?: ExtensionCache): Program {
         let program: Program;
         let files: SourceFile[] = [];
         let commonSourceDirectory: string;
@@ -1187,6 +1182,8 @@ namespace ts {
         // unconditionally set oldProgram to undefined to prevent it from being captured in closure
         oldProgram = undefined;
 
+        extensionCache = extensionCache || createExtensionCache(options, host);
+
         program = {
             getRootFileNames: () => rootNames,
             getSourceFile,
@@ -1209,7 +1206,13 @@ namespace ts {
             getSymbolCount: () => getDiagnosticsProducingTypeChecker().getSymbolCount(),
             getTypeCount: () => getDiagnosticsProducingTypeChecker().getTypeCount(),
             getFileProcessingDiagnostics: () => fileProcessingDiagnostics,
-            getResolvedTypeReferenceDirectives: () => resolvedTypeReferenceDirectives
+            getResolvedTypeReferenceDirectives: () => resolvedTypeReferenceDirectives,
+            getCompilerExtensions() {
+                return extensionCache.getCompilerExtensions();
+            },
+            getExtensionLoadingDiagnostics() {
+                return extensionCache.getExtensionLoadingDiagnostics();
+            },
         };
 
         verifyCompilerOptions();
@@ -1753,6 +1756,7 @@ namespace ts {
             const allDiagnostics: Diagnostic[] = [];
             addRange(allDiagnostics, fileProcessingDiagnostics.getGlobalDiagnostics());
             addRange(allDiagnostics, programDiagnostics.getGlobalDiagnostics());
+            allDiagnostics.push(...extensionCache.getExtensionLoadingDiagnostics());
             return sortAndDeduplicateDiagnostics(allDiagnostics);
         }
 
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 29ae2c60af165..251c22fd43ade 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -32,6 +32,7 @@ namespace ts {
         getMemoryUsage?(): number;
         exit(exitCode?: number): void;
         realpath?(path: string): string;
+        loadExtension?(name: string): any;
     }
 
     export interface FileWatcher {
@@ -548,6 +549,9 @@ namespace ts {
                 },
                 realpath(path: string): string {
                     return _fs.realpathSync(path);
+                },
+                loadExtension(name) {
+                    return require(name);
                 }
             };
             return nodeSystem;
diff --git a/src/compiler/tsc.ts b/src/compiler/tsc.ts
index 10538d0c009ee..9ecde9262f5f2 100644
--- a/src/compiler/tsc.ts
+++ b/src/compiler/tsc.ts
@@ -607,13 +607,18 @@ namespace ts {
             // First get and report any syntactic errors.
             diagnostics = program.getSyntacticDiagnostics();
 
+            // Count warnings/messages and ignore them for determining continued error reporting
+            const nonErrorCount = countWhere(diagnostics, d => d.category !== DiagnosticCategory.Error);
+
             // If we didn't have any syntactic errors, then also try getting the global and
             // semantic errors.
-            if (diagnostics.length === 0) {
-                diagnostics = program.getOptionsDiagnostics().concat(program.getGlobalDiagnostics());
+            if (diagnostics.length === nonErrorCount) {
+                diagnostics = diagnostics.concat(program.getOptionsDiagnostics().concat(program.getGlobalDiagnostics()));
+
+                const nonErrorCount = countWhere(diagnostics, d => d.category !== DiagnosticCategory.Error);
 
-                if (diagnostics.length === 0) {
-                    diagnostics = program.getSemanticDiagnostics();
+                if (diagnostics.length === nonErrorCount) {
+                    diagnostics = diagnostics.concat(program.getSemanticDiagnostics());
                 }
             }
 
diff --git a/src/compiler/tsconfig.json b/src/compiler/tsconfig.json
index cc9bfddcece78..9d98da46cd1a1 100644
--- a/src/compiler/tsconfig.json
+++ b/src/compiler/tsconfig.json
@@ -24,6 +24,7 @@
         "declarationEmitter.ts",
         "emitter.ts",
         "program.ts",
+        "extensions.ts",
         "commandLineParser.ts",
         "tsc.ts",
         "diagnosticInformationMap.generated.ts"
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 28ededebb0de1..dc750e2c9dff5 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2540,7 +2540,7 @@ namespace ts {
         length: number;
         messageText: string | DiagnosticMessageChain;
         category: DiagnosticCategory;
-        code: number;
+        code: number | string;
     }
 
     export enum DiagnosticCategory {
@@ -2633,6 +2633,7 @@ namespace ts {
         typeRoots?: string[];
         /*@internal*/ version?: boolean;
         /*@internal*/ watch?: boolean;
+        extensions?: string[] | Map<any>;
 
         [option: string]: CompilerOptionsValue | undefined;
     }
@@ -2966,6 +2967,14 @@ namespace ts {
          * This method is a companion for 'resolveModuleNames' and is used to resolve 'types' references to actual type declaration files
          */
         resolveTypeReferenceDirectives?(typeReferenceDirectiveNames: string[], containingFile: string): ResolvedTypeReferenceDirective[];
+
+        /**
+         * Delegates the loading of compiler extensions to the compiler host.
+         * The function should return the result of executing the code of an extension
+         * - its exported members. These members will be searched for objects who have been decorated with
+         * specific flags.
+         */
+        loadExtension?(extension: string): any;
     }
 
     export interface TextSpan {
diff --git a/src/harness/extensionRunner.ts b/src/harness/extensionRunner.ts
new file mode 100644
index 0000000000000..ca7ab8c4a6d9b
--- /dev/null
+++ b/src/harness/extensionRunner.ts
@@ -0,0 +1,492 @@
+///<reference path="harness.ts" />
+///<reference path="runnerbase.ts" />
+/// <reference path="fourslash.ts" />
+
+interface ExtensionTestConfig {
+    inputFiles: string[]; // Files from the source directory to include in the compilation
+    fourslashTest?: string; // File from the fourslash directory to test this compilation with
+    availableExtensions: string[]; // Extensions from the available directory to make available to the test
+    compilerOptions?: ts.CompilerOptions; // Optional compiler options to run with (usually at least "extensions" is specified)
+}
+
+type VirtualCompilationFunction = (files: string[], options: ts.CompilerOptions) => Harness.Compiler.CompilerResult;
+
+class ExtensionRunner extends RunnerBase {
+    private basePath = "tests/cases/extensions";
+    private scenarioPath = ts.combinePaths(this.basePath, "scenarios");
+    private extensionPath = ts.combinePaths(this.basePath, "available");
+    private sourcePath = ts.combinePaths(this.basePath, "source");
+    private fourslashPath = ts.combinePaths(this.basePath, "fourslash");
+    private extensionAPI: ts.Map<string> = {};
+    private extensions: ts.Map<ts.Map<string>> = {};
+    private virtualLib: ts.Map<string> = {};
+    private virtualFs: ts.Map<string> = {};
+
+    prettyPrintDiagnostic(diagnostic: ts.Diagnostic): string {
+        const message = ts.flattenDiagnosticMessageText(diagnostic.messageText, "\n");
+        if (diagnostic.file) {
+            const { line, character } = diagnostic.file.getLineAndCharacterOfPosition(diagnostic.start);
+            return `${diagnostic.file.fileName} (${line + 1},${character + 1}): ${message}`;
+        }
+        else {
+            return `!!!${message}`;
+        }
+    }
+
+    private innerCanonicalName = ts.createGetCanonicalFileName(true);
+    private getCanonicalFileName = (fileName: string) => ts.toPath(fileName, "/", this.innerCanonicalName);
+
+    loadSetIntoFsAt(set: ts.Map<string>, prefix: string) {
+        ts.Debug.assert(!!prefix, "Prefix must exist");
+        ts.Debug.assert(set !== this.virtualFs, "You cannot try to load the fs into itself.");
+
+        // Load a fileset at the given location, but exclude the 'lib' kind files from the added set (they'll be reloaded at the top level before compilation)
+        ts.forEachKey(set, key => ts.forEachKey(this.virtualLib, path => key === path) ? void 0 : void (this.virtualFs[this.getCanonicalFileName(`${prefix}/${key}`)] = set[key]));
+    }
+
+    loadSetIntoFs(set: ts.Map<string>) {
+        ts.Debug.assert(set !== this.virtualFs, "You cannot try to load the fs into itself.");
+        ts.forEachKey(set, key => void (this.virtualFs[this.getCanonicalFileName(key)] = set[key]));
+    }
+
+    private traces: string[] = [];
+    private mockHost: ts.CompilerHost = {
+        useCaseSensitiveFileNames: () => true,
+        getNewLine: () => "\n",
+        readFile: (path) => this.virtualFs[this.mockHost.getCanonicalFileName(path)],
+        writeFile: (path, content, foo, bar, baz) => {
+            this.virtualFs[this.mockHost.getCanonicalFileName(path)] = content;
+        },
+        fileExists: (path) => {
+            return !!this.virtualFs[this.mockHost.getCanonicalFileName(path)];
+        },
+        directoryExists: (path) => {
+            const fullPath = this.mockHost.getCanonicalFileName(path);
+            return ts.forEach(ts.getKeys(this.virtualFs), key => ts.startsWith(key, fullPath));
+        },
+        getCurrentDirectory(): string { return "/"; },
+        getSourceFile: (path, languageVersion, onError): ts.SourceFile => {
+            const fullPath = this.mockHost.getCanonicalFileName(path);
+            return ts.createSourceFile(fullPath, this.virtualFs[fullPath], languageVersion);
+        },
+        getDefaultLibLocation: () => "/lib/",
+        getDefaultLibFileName: (options) => {
+            return ts.combinePaths(this.mockHost.getDefaultLibLocation(), ts.getDefaultLibFileName(options));
+        },
+        getCanonicalFileName: this.getCanonicalFileName,
+        getDirectories: (path) => {
+            path = this.mockHost.getCanonicalFileName(path);
+            return ts.filter(ts.map(ts.filter(ts.getKeys(this.virtualFs),
+                fullpath => ts.startsWith(fullpath, path) && fullpath.substr(path.length, 1) === "/"),
+                    fullpath => fullpath.substr(path.length + 1).indexOf("/") >= 0 ? fullpath.substr(0, 1 + path.length + fullpath.substr(path.length + 1).indexOf("/")) : fullpath),
+                        fullpath => fullpath.lastIndexOf(".") === -1);
+        },
+        loadExtension: (path) => this.mockLoadExtension(path),
+        trace: (s) => {
+            this.traces.push(s);
+        }
+    };
+
+    mockLoadExtension(path: string) {
+        const fullPath = this.getCanonicalFileName(path);
+        const m = { exports: {} };
+        ((module, exports, require) => { eval(this.virtualFs[fullPath]); })(
+            m,
+            m.exports,
+            (name: string) => {
+                return this.mockLoadExtension(
+                    this.getCanonicalFileName(
+                        ts.resolveModuleName(name, fullPath, { module: ts.ModuleKind.CommonJS, moduleResolution: ts.ModuleResolutionKind.NodeJs }, this.mockHost, true).resolvedModule.resolvedFileName
+                    )
+                );
+            }
+        );
+        return m.exports;
+    }
+
+    makeLSMockAdapter(files: string[], options: ts.CompilerOptions, token?: ts.HostCancellationToken) {
+        const adapter = new Harness.LanguageService.NativeLanguageServiceAdapter(token, options);
+        // The host returned by the harness is _mostly_ suitable for use here
+        //    it just needs to be monkeypatched to load extensions, report directories, and canonicalize script paths
+        const host = adapter.getHost();
+        host.getDefaultLibFileName = () => "/lib/lib.d.ts";
+        host.getCurrentDirectory = () => "/";
+        (host as ts.LanguageServiceHost).loadExtension = (path) => this.mockLoadExtension(path);
+        (host as ts.LanguageServiceHost).useCaseSensitiveFileNames = () => true;
+        host.trace = (s) => {
+            this.traces.push(s);
+        };
+        host.getScriptInfo = (fileName: string) => {
+            fileName = this.getCanonicalFileName(fileName);
+            return ts.lookUp(host.fileNameToScript, fileName);
+        };
+        host.getDirectories = (s: string) => this.mockHost.getDirectories(s);
+        host.addScript = (fileName: string, content: string, isRootFile: boolean): void => {
+            const canonical = this.getCanonicalFileName(fileName);
+            host.fileNameToScript[canonical] = new Harness.LanguageService.ScriptInfo(canonical, content, isRootFile);
+        };
+        ts.forEach(files, file => {
+            host.addScript(file, this.virtualFs[file], looksLikeRootFile(file));
+        });
+
+        return adapter;
+
+        function looksLikeRootFile(file: string) {
+            return ts.endsWith(file, ".ts") && !ts.endsWith(file, ".d.ts") && (file.indexOf("node_modules") === -1);
+        }
+    }
+
+    makeMockLSHost(files: string[], options: ts.CompilerOptions) {
+        const adapter = this.makeLSMockAdapter(files, options);
+        return adapter.getHost();
+    };
+
+    getTraces(): string[] {
+        const traces = this.traces;
+        this.traces = [];
+        return traces.map(t => t.replace(/\([0-9\.e\+\-]+ ms\)$/, "(REDACTED ms)"));
+    }
+
+    languageServiceCompile(typescriptFiles: string[], options: ts.CompilerOptions): Harness.Compiler.CompilerResult {
+        const self = this;
+        const host = this.makeMockLSHost(ts.getKeys(this.virtualFs), options);
+        const service = ts.createLanguageService(host);
+        const fileResults: Harness.Compiler.GeneratedFile[] = [];
+
+        const diagnostics = ts.concatenate(ts.concatenate(
+            service.getProgramDiagnostics(),
+            ts.flatten(ts.map(typescriptFiles, fileName => service.getSyntacticDiagnostics(this.getCanonicalFileName(fileName))))),
+            ts.flatten(ts.map(typescriptFiles, fileName => service.getSemanticDiagnostics(this.getCanonicalFileName(fileName)))));
+
+        const emitResult = service.getProgram().emit(/*targetSourceFile*/undefined, writeFile);
+
+        const allDiagnostics = ts.sortAndDeduplicateDiagnostics(ts.concatenate(diagnostics, emitResult.diagnostics));
+
+        return new Harness.Compiler.CompilerResult(fileResults, allDiagnostics, /*program*/undefined, host.getCurrentDirectory(), emitResult.sourceMaps, this.getTraces());
+
+        function writeFile(fileName: string, code: string, writeByteOrderMark: boolean, onError: (message: string) => void, sourceFiles: ts.SourceFile[]) {
+            fileResults.push({
+                fileName,
+                writeByteOrderMark,
+                code
+            });
+            self.mockHost.writeFile(fileName, code, writeByteOrderMark, onError, sourceFiles);
+        }
+    }
+
+    programCompile(typescriptFiles: string[], options: ts.CompilerOptions): Harness.Compiler.CompilerResult {
+        const self = this;
+        const program = ts.createProgram(typescriptFiles, options, this.mockHost);
+        const fileResults: Harness.Compiler.GeneratedFile[] = [];
+        const diagnostics = ts.getPreEmitDiagnostics(program);
+        const emitResult = program.emit(/*targetSourceFile*/undefined, writeFile);
+
+        const allDiagnostics = ts.sortAndDeduplicateDiagnostics(ts.concatenate(diagnostics, emitResult.diagnostics));
+
+        return new Harness.Compiler.CompilerResult(fileResults, allDiagnostics, /*program*/undefined, this.mockHost.getCurrentDirectory(), emitResult.sourceMaps, this.getTraces());
+        function writeFile(fileName: string, code: string, writeByteOrderMark: boolean, onError: (message: string) => void, sourceFiles: ts.SourceFile[]) {
+            fileResults.push({
+                fileName,
+                writeByteOrderMark,
+                code
+            });
+            self.mockHost.writeFile(fileName, code, writeByteOrderMark, onError, sourceFiles);
+        }
+    }
+
+    compile(fileset: ts.Map<string>, options: ts.CompilerOptions, compileFunc: VirtualCompilationFunction): Harness.Compiler.CompilerResult {
+        this.loadSetIntoFs(this.virtualLib);
+        this.loadSetIntoFs(fileset);
+
+        // Consider all TS files in the passed fileset as the root files, but not any under a node_modules folder
+        const typescriptFiles = ts.filter(ts.getKeys(fileset), name => ts.endsWith(name, ".ts") && !(name.indexOf("node_modules") >= 0));
+        return compileFunc(typescriptFiles, options);
+    }
+
+    buildMap(compileFunc: VirtualCompilationFunction, map: ts.Map<string>, out: ts.Map<string>, compilerOptions?: ts.CompilerOptions, shouldError?: boolean): Harness.Compiler.CompilerResult {
+        const results = this.compile(map, compilerOptions ? compilerOptions : { module: ts.ModuleKind.CommonJS, declaration: true }, compileFunc);
+        const diagnostics = results.errors;
+        if (shouldError && diagnostics && diagnostics.length) {
+            for (let i = 0; i < diagnostics.length; i++) {
+                console.log(this.prettyPrintDiagnostic(diagnostics[i]));
+            }
+            throw new Error("Compiling test harness extension API code resulted in errors.");
+        }
+        ts.copyMap(this.virtualFs, out);
+        this.virtualFs = {};
+        return results;
+    }
+
+    private loadExtensions() {
+        this.extensionAPI = {
+            "package.json": Harness.IO.readFile(ts.combinePaths(this.extensionPath, "extension-api/package.json")),
+            "index.ts": Harness.IO.readFile(ts.combinePaths(this.extensionPath, "extension-api/index.ts")),
+        };
+        this.buildMap((str, opts) => this.programCompile(str, opts), this.extensionAPI, this.extensionAPI, { module: ts.ModuleKind.CommonJS, declaration: true }, /*shouldError*/true);
+
+        ts.forEach(Harness.IO.getDirectories(this.extensionPath), path => {
+            if (path === "extension-api" || path === "typescript") return; // Since these are dependencies of every actual test extension, we handle them specially
+            const packageDir = ts.combinePaths(this.extensionPath, path);
+            const extensionFileset: ts.Map<string> = {};
+            const extensionFiles = this.enumerateFiles(packageDir, /*regex*/ undefined, { recursive: true });
+            ts.forEach(extensionFiles, name => {
+                const shortName = name.substring(packageDir.length + 1);
+                extensionFileset[shortName] = Harness.IO.readFile(name);
+            });
+            this.loadSetIntoFsAt(this.extensionAPI, "/node_modules/extension-api");
+
+            this.buildMap((str, opts) => this.programCompile(str, opts), extensionFileset, extensionFileset, { module: ts.ModuleKind.CommonJS, declaration: true }, /*shouldError*/true);
+            this.extensions[path] = extensionFileset;
+        });
+    }
+
+    constructor() {
+        super();
+        const {content: libContent} = Harness.getDefaultLibraryFile(Harness.IO);
+        const tsLibContents = Harness.IO.readFile("built/local/typescript.d.ts");
+        this.virtualLib = {
+            "/lib/lib.d.ts": libContent,
+            "/node_modules/typescript/index.d.ts": tsLibContents
+        };
+        this.loadExtensions();
+    }
+
+    kind(): "extension" {
+        return "extension";
+    }
+
+    enumerateTestFiles(): string[] {
+        return this.enumerateFiles(this.scenarioPath, /\.json$/, { recursive: true });
+    }
+
+    /** Setup the runner's tests so that they are ready to be executed by the harness
+     *  The first test should be a describe/it block that sets up the harness's compiler instance appropriately
+     */
+    public initializeTests(): void {
+        describe("Compiler Extensions", () => {
+            if (this.tests.length === 0) {
+                const testFiles = this.enumerateTestFiles();
+                testFiles.forEach(fn => {
+                    this.runTest(fn);
+                });
+            }
+            else {
+                this.tests.forEach(test => this.runTest(test));
+            }
+        });
+    }
+
+    getByteOrderMarkText(file: Harness.Compiler.GeneratedFile): string {
+        return file.writeByteOrderMark ? "\u00EF\u00BB\u00BF" : "";
+    }
+
+    private compileTargets: [string, VirtualCompilationFunction][] = [["CompilerHost", (str, opts) => this.programCompile(str, opts)], ["LanguageServiceHost", (str, opts) => this.languageServiceCompile(str, opts)]];
+    /**
+     * Extensions tests are complete end-to-end tests with multiple compilations to prepare a test
+     * 
+     * Tests need to be:
+     *  Run under both `compilerHost` and `languageServiceHost` environments
+     *   - When under LSHost, verify all fourslash test-type results included in the test
+     *   - Verify output baseline
+     *   - Verify error baseline
+     *   - Verify sourcemaps if need be
+     *   - Verify traces if need be
+     */
+    private runTest(caseName: string) {
+        const caseNameNoExtension = caseName.replace(/\.json$/, "");
+        describe(caseNameNoExtension, () => {
+            let shortCasePath: string;
+            let testConfigText: string;
+            let testConfig: ExtensionTestConfig;
+            let inputSources: ts.Map<string>;
+            let inputTestFiles: Harness.Compiler.TestFile[];
+            before(() => {
+                shortCasePath = caseName.substring(this.scenarioPath.length + 1).replace(/\.json$/, "");
+                testConfigText = Harness.IO.readFile(caseName);
+                testConfig = JSON.parse(testConfigText);
+                inputSources = {};
+                inputTestFiles = [];
+                ts.forEach(testConfig.inputFiles, name => {
+                    inputSources[name] = Harness.IO.readFile(ts.combinePaths(this.sourcePath, name));
+                    inputTestFiles.push({
+                        unitName: this.getCanonicalFileName(name),
+                        content: inputSources[name]
+                    });
+                });
+            });
+
+            after(() => {
+                shortCasePath = undefined;
+                testConfigText = undefined;
+                testConfig = undefined;
+                inputSources = undefined;
+                inputTestFiles = undefined;
+            });
+
+            ts.forEach(this.compileTargets, ([name, compileCb]) => {
+                describe(`${name}`, () => {
+                    let sources: ts.Map<string>;
+                    let result: Harness.Compiler.CompilerResult;
+                    before(() => {
+                        this.traces = []; // Clear out any traces from tests which made traces, but didn't specify traceResolution
+                        this.virtualFs = {}; // In case a fourslash test was run last (which doesn't clear FS on end like buildMap does), clear the FS
+                        sources = {};
+                        ts.copyMap(inputSources, sources);
+                        ts.forEach(testConfig.availableExtensions, ext => this.loadSetIntoFsAt(this.extensions[ext], `/node_modules/${ext}`));
+                        result = this.buildMap(compileCb, sources, sources, testConfig.compilerOptions, /*shouldError*/false);
+                    });
+
+                    after(() => {
+                        sources = undefined;
+                        result = undefined;
+                    });
+
+                    const errorsTestName = `Correct errors`;
+                    it(errorsTestName, () => {
+                        Harness.Baseline.runBaseline(errorsTestName, `${name}/${shortCasePath}.errors.txt`, () => {
+                            /* tslint:disable:no-null-keyword */
+                            if (result.errors.length === 0) return null;
+                            /* tslint:enable:no-null-keyword */
+                            return Harness.Compiler.getErrorBaseline(inputTestFiles, result.errors);
+                        });
+                    });
+
+                    const traceTestName = `Correct traces`;
+                    it(traceTestName, () => {
+                        if (!(testConfig.compilerOptions.traceResolution)) {
+                            return;
+                        }
+                        Harness.Baseline.runBaseline(traceTestName, `${name}/${shortCasePath}.trace.txt`, (): string => {
+                            return (result.traceResults || []).join("\n");
+                        });
+                    });
+
+                    const sourcemapTestName = `Correct sourcemap content`;
+                    it(sourcemapTestName, () => {
+                        if (!(testConfig.compilerOptions.sourceMap || testConfig.compilerOptions.inlineSourceMap)) {
+                            return;
+                        }
+                        Harness.Baseline.runBaseline(sourcemapTestName, `${name}/${shortCasePath}.sourcemap.txt`, () => {
+                            const record = result.getSourceMapRecord();
+                            if (testConfig.compilerOptions.noEmitOnError && result.errors.length !== 0 && record === undefined) {
+                                // Because of the noEmitOnError option no files are created. We need to return null because baselining isn't required.
+                                /* tslint:disable:no-null-keyword */
+                                return null;
+                                /* tslint:enable:no-null-keyword */
+                            }
+                            return record;
+                        });
+                    });
+
+                    const sourcemapOutputTestName = `Correct sourcemap output`;
+                    it(sourcemapOutputTestName, () => {
+                        if (testConfig.compilerOptions.inlineSourceMap) {
+                            if (result.sourceMaps.length > 0) {
+                                throw new Error("No sourcemap files should be generated if inlineSourceMaps was set.");
+                            }
+                            return;
+                        }
+                        else if (!testConfig.compilerOptions.sourceMap) {
+                            return;
+                        }
+                        if (result.sourceMaps.length !== result.files.length) {
+                            throw new Error("Number of sourcemap files should be same as js files.");
+                        }
+
+                        Harness.Baseline.runBaseline(sourcemapOutputTestName, `${name}/${shortCasePath}.js.map`, () => {
+                            if (testConfig.compilerOptions.noEmitOnError && result.errors.length !== 0 && result.sourceMaps.length === 0) {
+                                // We need to return null here or the runBaseLine will actually create a empty file.
+                                // Baselining isn't required here because there is no output.
+                                /* tslint:disable:no-null-keyword */
+                                return null;
+                                /* tslint:enable:no-null-keyword */
+                            }
+
+                            let sourceMapCode = "";
+                            for (let i = 0; i < result.sourceMaps.length; i++) {
+                                sourceMapCode += "//// [" + Harness.Path.getFileName(result.sourceMaps[i].fileName) + "]\r\n";
+                                sourceMapCode += this.getByteOrderMarkText(result.sourceMaps[i]);
+                                sourceMapCode += result.sourceMaps[i].code;
+                            }
+
+                            return sourceMapCode;
+                        });
+                    });
+
+                    const emitOutputTestName = `Correct emit (JS/DTS)`;
+                    it(emitOutputTestName, () => {
+                        if (!ts.forEach(testConfig.inputFiles, name => !ts.endsWith(name, ".d.ts"))) {
+                            return;
+                        }
+                        if (!testConfig.compilerOptions.noEmit && result.files.length === 0 && result.errors.length === 0) {
+                            throw new Error("Expected at least one js file to be emitted or at least one error to be created.");
+                        }
+
+                        // check js output
+                        Harness.Baseline.runBaseline(emitOutputTestName, `${name}/${shortCasePath}.js`, () => {
+                            let tsCode = "";
+                            const tsSources = inputTestFiles;
+                            if (tsSources.length > 1) {
+                                tsCode += "//// [" + caseNameNoExtension + "] ////\r\n\r\n";
+                            }
+                            for (let i = 0; i < tsSources.length; i++) {
+                                tsCode += "//// [" + Harness.Path.getFileName(tsSources[i].unitName) + "]\r\n";
+                                tsCode += tsSources[i].content + (i < (tsSources.length - 1) ? "\r\n" : "");
+                            }
+
+                            let jsCode = "";
+                            for (let i = 0; i < result.files.length; i++) {
+                                jsCode += "//// [" + Harness.Path.getFileName(result.files[i].fileName) + "]\r\n";
+                                jsCode += this.getByteOrderMarkText(result.files[i]);
+                                jsCode += result.files[i].code;
+                            }
+
+                            if (result.declFilesCode.length > 0) {
+                                jsCode += "\r\n\r\n";
+                                for (let i = 0; i < result.declFilesCode.length; i++) {
+                                    jsCode += "//// [" + Harness.Path.getFileName(result.declFilesCode[i].fileName) + "]\r\n";
+                                    jsCode += this.getByteOrderMarkText(result.declFilesCode[i]);
+                                    jsCode += result.declFilesCode[i].code;
+                                }
+                            }
+
+                            if (jsCode.length > 0) {
+                                return tsCode + "\r\n\r\n" + jsCode;
+                            }
+                            else {
+                                /* tslint:disable:no-null-keyword */
+                                return null;
+                                /* tslint:enable:no-null-keyword */
+                            }
+                        });
+                    });
+                });
+            });
+
+            it("passes fourslash verification", () => {
+                if (testConfig.fourslashTest) {
+                    this.virtualFs = {};
+                    const testFile = `${this.fourslashPath}/${testConfig.fourslashTest}`;
+                    let testFileContents = Harness.IO.readFile(testFile);
+                    testFileContents = testFileContents.replace(`/// <reference path="../../fourslash/fourslash.ts" />`, "");
+                    const testContent = [`/// <reference path="fourslash.ts" />`, ""];
+                    ts.forEach(inputTestFiles, testFile => {
+                        testContent.push(`// @Filename: ${testFile.unitName.substring(1)}`); // Drop leading /
+                        testContent.push(...testFile.content.split("\n").map(s => `////${s}`));
+                    });
+                    testContent.push("// @Filename: tsconfig.json");
+                    testContent.push(`////${JSON.stringify(testConfig.compilerOptions)}`);
+                    testContent.push(testFileContents);
+                    const finishedTestContent = testContent.join("\n");
+
+                    this.loadSetIntoFs(this.virtualLib);
+                    ts.forEach(testConfig.availableExtensions, ext => this.loadSetIntoFsAt(this.extensions[ext], `/node_modules/${ext}`));
+
+                    const adapterFactory = (token: ts.HostCancellationToken) => this.makeLSMockAdapter(ts.getKeys(this.virtualFs), testConfig.compilerOptions, token);
+
+                    FourSlash.runFourSlashTestContent(shortCasePath, adapterFactory, finishedTestContent, testFile);
+                }
+            });
+        });
+    }
+}
\ No newline at end of file
diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index a42abbbc60909..14f16eb09f2d6 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -242,7 +242,7 @@ namespace FourSlash {
             }
         }
 
-        constructor(private basePath: string, private testType: FourSlashTestType, public testData: FourSlashData) {
+        constructor(private basePath: string, private testType: FourSlashTestType | ((token: ts.HostCancellationToken) => Harness.LanguageService.LanguageServiceAdapter), public testData: FourSlashData) {
             // Create a new Services Adapter
             this.cancellationToken = new TestCancellationToken();
             const compilationOptions = convertGlobalOptionsToCompilerOptions(this.testData.globalOptions);
@@ -250,7 +250,13 @@ namespace FourSlash {
                 compilationOptions.typeRoots = compilationOptions.typeRoots.map(p => ts.getNormalizedAbsolutePath(p, this.basePath));
             }
 
-            const languageServiceAdapter = this.getLanguageServiceAdapter(testType, this.cancellationToken, compilationOptions);
+            let languageServiceAdapter: Harness.LanguageService.LanguageServiceAdapter;
+            if (typeof testType === "number") {
+                languageServiceAdapter = this.getLanguageServiceAdapter(testType, this.cancellationToken, compilationOptions);
+            }
+            else {
+                languageServiceAdapter = testType(this.cancellationToken);
+            }
             this.languageServiceAdapterHost = languageServiceAdapter.getHost();
             this.languageService = languageServiceAdapter.getLanguageService();
 
@@ -2266,7 +2272,7 @@ namespace FourSlash {
         runFourSlashTestContent(basePath, testType, content, fileName);
     }
 
-    export function runFourSlashTestContent(basePath: string, testType: FourSlashTestType, content: string, fileName: string): void {
+    export function runFourSlashTestContent(basePath: string, testType: FourSlashTestType | ((token: ts.HostCancellationToken) => Harness.LanguageService.LanguageServiceAdapter), content: string, fileName: string): void {
         // Parse out the files and their metadata
         const testData = parseTestData(basePath, content, fileName);
 
diff --git a/src/harness/harness.ts b/src/harness/harness.ts
index f27e7e1c1749a..4579dbe4aae59 100644
--- a/src/harness/harness.ts
+++ b/src/harness/harness.ts
@@ -1209,6 +1209,11 @@ namespace Harness {
             return normalized;
         }
 
+        function getDiagnosticCodeString(code: string | number) {
+            if (typeof code === "number") return `TS${code}`;
+            return code;
+        }
+
         export function minimalDiagnosticsToString(diagnostics: ts.Diagnostic[]) {
             return ts.formatDiagnostics(diagnostics, { getCanonicalFileName, getCurrentDirectory: () => "", getNewLine: () => Harness.IO.newLine() });
         }
@@ -1226,7 +1231,7 @@ namespace Harness {
                     .split("\n")
                     .map(s => s.length > 0 && s.charAt(s.length - 1) === "\r" ? s.substr(0, s.length - 1) : s)
                     .filter(s => s.length > 0)
-                    .map(s => "!!! " + ts.DiagnosticCategory[error.category].toLowerCase() + " TS" + error.code + ": " + s);
+                    .map(s => "!!! " + ts.DiagnosticCategory[error.category].toLowerCase() + " " + getDiagnosticCodeString(error.code) + ": " + s);
                 errLines.forEach(e => outputLines.push(e));
 
                 // do not count errors from lib.d.ts here, they are computed separately as numLibraryDiagnostics
diff --git a/src/harness/harnessLanguageService.ts b/src/harness/harnessLanguageService.ts
index d7ed04b627f4f..fdd2f6602ecb5 100644
--- a/src/harness/harnessLanguageService.ts
+++ b/src/harness/harnessLanguageService.ts
@@ -123,7 +123,7 @@ namespace Harness.LanguageService {
     }
 
     export class LanguageServiceAdapterHost {
-        protected fileNameToScript: ts.Map<ScriptInfo> = {};
+        public fileNameToScript: ts.Map<ScriptInfo> = {};
 
         constructor(protected cancellationToken = DefaultHostCancellationToken.Instance,
                     protected settings = ts.getDefaultCompilerOptions()) {
@@ -366,6 +366,9 @@ namespace Harness.LanguageService {
         getCompilerOptionsDiagnostics(): ts.Diagnostic[] {
             return unwrapJSONCallResult(this.shim.getCompilerOptionsDiagnostics());
         }
+        getProgramDiagnostics(): ts.Diagnostic[] {
+            return unwrapJSONCallResult(this.shim.getProgramDiagnostics());
+        }
         getSyntacticClassifications(fileName: string, span: ts.TextSpan): ts.ClassifiedSpan[] {
             return unwrapJSONCallResult(this.shim.getSyntacticClassifications(fileName, span.start, span.length));
         }
diff --git a/src/harness/runner.ts b/src/harness/runner.ts
index 4b1945f5baa33..942ee516e9476 100644
--- a/src/harness/runner.ts
+++ b/src/harness/runner.ts
@@ -17,6 +17,7 @@
 /// <reference path="compilerRunner.ts" />
 /// <reference path="fourslashRunner.ts" />
 /// <reference path="projectsRunner.ts" />
+/// <reference path="extensionRunner.ts" />
 /// <reference path="rwcRunner.ts" />
 /// <reference path="harness.ts" />
 
@@ -58,6 +59,8 @@ function createRunner(kind: TestRunnerKind): RunnerBase {
             return new RWCRunner();
         case "test262":
             return new Test262BaselineRunner();
+        case "extension":
+            return new ExtensionRunner();
     }
 }
 
@@ -155,6 +158,9 @@ if (testConfigContent !== "") {
                 case "test262":
                     runners.push(new Test262BaselineRunner());
                     break;
+                case "extension":
+                    runners.push(new ExtensionRunner());
+                    break;
             }
         }
     }
@@ -176,6 +182,9 @@ if (runners.length === 0) {
     runners.push(new FourSlashRunner(FourSlashTestType.ShimsWithPreprocess));
     runners.push(new FourSlashRunner(FourSlashTestType.Server));
     // runners.push(new GeneratedFourslashRunner());
+
+    // extension
+    runners.push(new ExtensionRunner());
 }
 
 if (taskConfigsFolder) {
diff --git a/src/harness/runnerbase.ts b/src/harness/runnerbase.ts
index 346382b7a5721..35d345463fde7 100644
--- a/src/harness/runnerbase.ts
+++ b/src/harness/runnerbase.ts
@@ -1,7 +1,7 @@
 /// <reference path="harness.ts" />
 
 
-type TestRunnerKind = CompilerTestKind | FourslashTestKind | "project" | "rwc" | "test262";
+type TestRunnerKind = CompilerTestKind | FourslashTestKind | "project" | "rwc" | "test262" | "extension";
 type CompilerTestKind = "conformance" | "compiler";
 type FourslashTestKind = "fourslash" | "fourslash-shims" | "fourslash-shims-pp" | "fourslash-server";
 
diff --git a/src/server/client.ts b/src/server/client.ts
index f04dbd8dc0253..d12c50459b6fe 100644
--- a/src/server/client.ts
+++ b/src/server/client.ts
@@ -395,6 +395,10 @@ namespace ts.server {
         }
 
         getCompilerOptionsDiagnostics(): Diagnostic[] {
+            return this.getProgramDiagnostics();
+        }
+
+        getProgramDiagnostics(): Diagnostic[] {
             throw new Error("Not Implemented Yet.");
         }
 
diff --git a/src/server/editorServices.ts b/src/server/editorServices.ts
index 6e9adad7f472f..acc12116ecb57 100644
--- a/src/server/editorServices.ts
+++ b/src/server/editorServices.ts
@@ -1150,7 +1150,7 @@ namespace ts.server {
                     info.setFormatOptions(this.getFormatCodeOptions());
                     this.filenameToScriptInfo[fileName] = info;
                     if (!info.isOpen) {
-                        info.fileWatcher = this.host.watchFile(fileName, _ => { this.watchedFileChanged(fileName); });
+                        info.fileWatcher = this.host.watchFile && this.host.watchFile(fileName, _ => { this.watchedFileChanged(fileName); });
                     }
                 }
             }
diff --git a/src/services/services.ts b/src/services/services.ts
index ab1e30a44a607..55ce4efd6693a 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -1167,6 +1167,8 @@ namespace ts {
         resolveTypeReferenceDirectives?(typeDirectiveNames: string[], containingFile: string): ResolvedTypeReferenceDirective[];
         directoryExists?(directoryName: string): boolean;
         getDirectories?(directoryName: string): string[];
+
+        loadExtension?(path: string): any;
     }
 
     //
@@ -1179,10 +1181,13 @@ namespace ts {
         getSyntacticDiagnostics(fileName: string): Diagnostic[];
         getSemanticDiagnostics(fileName: string): Diagnostic[];
 
-        // TODO: Rename this to getProgramDiagnostics to better indicate that these are any
-        // diagnostics present for the program level, and not just 'options' diagnostics.
+        /**
+         * @deprecated Use getProgramDiagnostics instead.
+         */
         getCompilerOptionsDiagnostics(): Diagnostic[];
 
+        getProgramDiagnostics(): Diagnostic[];
+
         /**
          * @deprecated Use getEncodedSyntacticClassifications instead.
          */
@@ -1246,6 +1251,86 @@ namespace ts {
         dispose(): void;
     }
 
+    export interface LanguageServiceProvider {
+        // Overrides
+
+        // A plugin can implement one of the override methods to replace the results that would
+        // be returned by the TypeScript language service. If a plugin returns a defined result
+        // (that is, is not undefined) then that result is used instead of invoking the
+        // corresponding TypeScript method. If multiple plugins are registered, they are
+        // consulted in the order they are returned from the program. The first defined result
+        // returned by a plugin is used and no other plugin overrides are consulted.
+
+        getProgramDiagnostics?(): Diagnostic[];
+        getSyntacticDiagnostics?(fileName: string): Diagnostic[];
+        getSemanticDiagnostics?(fileName: string): Diagnostic[];
+        getEncodedSyntacticClassifications?(fileName: string, span: TextSpan): Classifications;
+        getEncodedSemanticClassifications?(fileName: string, span: TextSpan): Classifications;
+        getCompletionsAtPosition?(fileName: string, position: number): CompletionInfo;
+        getCompletionEntryDetails?(fileName: string, position: number, entryName: string): CompletionEntryDetails;
+        getQuickInfoAtPosition?(fileName: string, position: number): QuickInfo;
+        getNameOrDottedNameSpan?(fileName: string, startPos: number, endPos: number): TextSpan;
+        getBreakpointStatementAtPosition?(fileName: string, position: number): TextSpan;
+        getSignatureHelpItems?(fileName: string, position: number): SignatureHelpItems;
+        getRenameInfo?(fileName: string, position: number): RenameInfo;
+        findRenameLocations?(fileName: string, position: number, findInStrings: boolean, findInComments: boolean): RenameLocation[];
+        getDefinitionAtPosition?(fileName: string, position: number): DefinitionInfo[];
+        getTypeDefinitionAtPosition?(fileName: string, position: number): DefinitionInfo[];
+        getReferencesAtPosition?(fileName: string, position: number): ReferenceEntry[];
+        findReferences?(fileName: string, position: number): ReferencedSymbol[];
+        getDocumentHighlights?(fileName: string, position: number, filesToSearch: string[]): DocumentHighlights[];
+        getNavigateToItems?(searchValue: string, maxResultCount: number): NavigateToItem[];
+        getNavigationBarItems?(fileName: string): NavigationBarItem[];
+        getOutliningSpans?(fileName: string): OutliningSpan[];
+        getTodoComments?(fileName: string, descriptors: TodoCommentDescriptor[]): TodoComment[];
+        getBraceMatchingAtPosition?(fileName: string, position: number): TextSpan[];
+        getIndentationAtPosition?(fileName: string, position: number, options: EditorOptions): number;
+        getFormattingEditsForRange?(fileName: string, start: number, end: number, options: FormatCodeOptions): TextChange[];
+        getFormattingEditsForDocument?(fileName: string, options: FormatCodeOptions): TextChange[];
+        getFormattingEditsAfterKeystroke?(fileName: string, position: number, key: string, options: FormatCodeOptions): TextChange[];
+        getDocCommentTemplateAtPosition?(fileName: string, position: number): TextInsertion;
+
+        // Filters
+
+        // A plugin can implement one of the filter methods to augment, extend or modify a result
+        // prior to the host receiving it. The TypeScript language service is invoked and the
+        // result is passed to the plugin as the value of the previous parameter. If more than one
+        // plugin is registered, the plugins are consulted in the order they are returned from the
+        // program. The value passed in as previous is the result returned by the prior plugin. If a
+        // plugin returns undefined, the result passed in as previous is used and the undefined
+        // result is ignored. All plugins are consulted before the result is returned to the host.
+        // If a plugin overrides behavior of the method, no filter methods are consulted.
+
+        getProgramDiagnosticsFilter?(previous: Diagnostic[]): Diagnostic[];
+        getSyntacticDiagnosticsFilter?(fileName: string, previous: Diagnostic[]): Diagnostic[];
+        getSemanticDiagnosticsFilter?(fileName: string, previous: Diagnostic[]): Diagnostic[];
+        getEncodedSyntacticClassificationsFilter?(fileName: string, span: TextSpan, previous: Classifications): Classifications;
+        getEncodedSemanticClassificationsFilter?(fileName: string, span: TextSpan, previous: Classifications): Classifications;
+        getCompletionsAtPositionFilter?(fileName: string, position: number, previous: CompletionInfo): CompletionInfo;
+        getCompletionEntryDetailsFilter?(fileName: string, position: number, entryName: string, previous: CompletionEntryDetails): CompletionEntryDetails;
+        getQuickInfoAtPositionFilter?(fileName: string, position: number, previous: QuickInfo): QuickInfo;
+        getNameOrDottedNameSpanFilter?(fileName: string, startPos: number, endPos: number, previous: TextSpan): TextSpan;
+        getBreakpointStatementAtPositionFilter?(fileName: string, position: number, previous: TextSpan): TextSpan;
+        getSignatureHelpItemsFilter?(fileName: string, position: number, previous: SignatureHelpItems): SignatureHelpItems;
+        getRenameInfoFilter?(fileName: string, position: number, previous: RenameInfo): RenameInfo;
+        findRenameLocationsFilter?(fileName: string, position: number, findInStrings: boolean, findInComments: boolean, previous: RenameLocation[]): RenameLocation[];
+        getDefinitionAtPositionFilter?(fileName: string, position: number, previous: DefinitionInfo[]): DefinitionInfo[];
+        getTypeDefinitionAtPositionFilter?(fileName: string, position: number, previous: DefinitionInfo[]): DefinitionInfo[];
+        getReferencesAtPositionFilter?(fileName: string, position: number, previous: ReferenceEntry[]): ReferenceEntry[];
+        findReferencesFilter?(fileName: string, position: number, previous: ReferencedSymbol[]): ReferencedSymbol[];
+        getDocumentHighlightsFilter?(fileName: string, position: number, filesToSearch: string[], previous: DocumentHighlights[]): DocumentHighlights[];
+        getNavigateToItemsFilter?(searchValue: string, maxResultCount: number, previous: NavigateToItem[]): NavigateToItem[];
+        getNavigationBarItemsFilter?(fileName: string, previous: NavigationBarItem[]): NavigationBarItem[];
+        getOutliningSpansFilter?(fileName: string, previous: OutliningSpan[]): OutliningSpan[];
+        getTodoCommentsFilter?(fileName: string, descriptors: TodoCommentDescriptor[], previous: TodoComment[]): TodoComment[];
+        getBraceMatchingAtPositionFilter?(fileName: string, position: number, previous: TextSpan[]): TextSpan[];
+        getIndentationAtPositionFilter?(fileName: string, position: number, options: EditorOptions, previous: number): number;
+        getFormattingEditsForRangeFilter?(fileName: string, start: number, end: number, options: FormatCodeOptions, previous: TextChange[]): TextChange[];
+        getFormattingEditsForDocumentFilter?(fileName: string, options: FormatCodeOptions, previous: TextChange[]): TextChange[];
+        getFormattingEditsAfterKeystrokeFilter?(fileName: string, position: number, key: string, options: FormatCodeOptions, previous: TextChange[]): TextChange[];
+        getDocCommentTemplateAtPositionFilter?(fileName: string, position: number, previous: TextInsertion): TextInsertion;
+    }
+
     export interface Classifications {
         spans: number[];
         endOfLineState: EndOfLineState;
@@ -1826,6 +1911,7 @@ namespace ts {
         version: string;
         scriptSnapshot: IScriptSnapshot;
         scriptKind: ScriptKind;
+        isRoot: boolean;
     }
 
     interface DocumentRegistryEntry {
@@ -1902,7 +1988,7 @@ namespace ts {
             // Initialize the list with the root file names
             const rootFileNames = host.getScriptFileNames();
             for (const fileName of rootFileNames) {
-                this.createEntry(fileName, toPath(fileName, this.currentDirectory, getCanonicalFileName));
+                this.createEntry(fileName, toPath(fileName, this.currentDirectory, getCanonicalFileName), /*isRoot*/true);
             }
 
             // store the compilation settings
@@ -1913,7 +1999,7 @@ namespace ts {
             return this._compilationSettings;
         }
 
-        private createEntry(fileName: string, path: Path) {
+        private createEntry(fileName: string, path: Path, isRoot: boolean) {
             let entry: HostFileInformation;
             const scriptSnapshot = this.host.getScriptSnapshot(fileName);
             if (scriptSnapshot) {
@@ -1921,7 +2007,8 @@ namespace ts {
                     hostFileName: fileName,
                     version: this.host.getScriptVersion(fileName),
                     scriptSnapshot: scriptSnapshot,
-                    scriptKind: getScriptKind(fileName, this.host)
+                    scriptKind: getScriptKind(fileName, this.host),
+                    isRoot
                 };
             }
 
@@ -1945,14 +2032,14 @@ namespace ts {
         public getOrCreateEntryByPath(fileName: string, path: Path): HostFileInformation {
             return this.contains(path)
                 ? this.getEntry(path)
-                : this.createEntry(fileName, path);
+                : this.createEntry(fileName, path, /*isRoot*/false);
         }
 
         public getRootFileNames(): string[] {
             const fileNames: string[] = [];
 
             this.fileNameToEntry.forEachValue((path, value) => {
-                if (value) {
+                if (value && value.isRoot) {
                     fileNames.push(value.hostFileName);
                 }
             });
@@ -3019,11 +3106,84 @@ namespace ts {
     }
 
     export function createLanguageService(host: LanguageServiceHost,
+        documentRegistry: DocumentRegistry = createDocumentRegistry(host.useCaseSensitiveFileNames && host.useCaseSensitiveFileNames(), host.getCurrentDirectory())): LanguageService {
+            const baseService = createUnextendedLanguageService(host, documentRegistry);
+            const extensions = baseService.getProgram().getCompilerExtensions()["language-service"];
+            const instantiatedExtensions = map(extensions, extension => new extension.ctor({ ts, host, service: baseService, registry: documentRegistry, args: extension.args }));
+            const extensionCount = instantiatedExtensions && instantiatedExtensions.length;
+
+            function wrap(key: string): Function {
+                if (extensionCount) {
+                    return (...args: any[]) => {
+                        let result: any;
+                        for (let i = 0; i < extensionCount; i++) {
+                            const extension = instantiatedExtensions[i];
+                            if ((extension as any)[key]) {
+                                const temp = (extension as any)[key](...args);
+                                if (temp !== undefined) {
+                                    result = temp;
+                                    break;
+                                }
+                            }
+                        }
+                        result = result !== undefined ? result : (baseService as any)[key](...args);
+                        const filterKey = `${key}Filter`;
+                        for (let i = 0; i < extensionCount; i++) {
+                            const extension = instantiatedExtensions[i];
+                            if ((extension as any)[filterKey]) {
+                                const temp = (extension as any)[filterKey](...args, result);
+                                if (temp !== undefined) {
+                                    result = temp;
+                                }
+                            }
+                        }
+                        return result;
+                    };
+                }
+                return (baseService as any)[key];
+            }
+
+            function buildWrappedService(underlyingMembers: Map<any>, wrappedMembers: string[]): LanguageService {
+                // Add wrapped members to map
+                forEach(wrappedMembers, member => {
+                    underlyingMembers[member] = wrap(member);
+                });
+                // Map getProgramDiagnostics to deprecated getCompilerOptionsDiagnostics
+                underlyingMembers["getCompilerOptionsDiagnostics"] = underlyingMembers["getProgramDiagnostics"];
+                return underlyingMembers as LanguageService;
+            }
+
+            return buildWrappedService({
+                cleanupSemanticCache: () => baseService.cleanupSemanticCache(),
+                getSyntacticClassifications: (fileName: string, span: TextSpan) => baseService.getSyntacticClassifications(fileName, span),
+                getSemanticClassifications: (fileName: string, span: TextSpan) => baseService.getSemanticClassifications(fileName, span),
+                getOccurrencesAtPosition: (fileName: string, position: number) => baseService.getOccurrencesAtPosition(fileName, position),
+                isValidBraceCompletionAtPosition: (fileName: string, pos: number, openingBrace: number) => baseService.isValidBraceCompletionAtPosition(fileName, pos, openingBrace),
+                getEmitOutput: (fileName: string) => baseService.getEmitOutput(fileName),
+                getProgram: () => baseService.getProgram(),
+                getNonBoundSourceFile: (fileName: string) => baseService.getNonBoundSourceFile(fileName),
+                dispose: () => baseService.dispose(),
+            }, [
+                "getSyntacticDiagnostics", "getSemanticDiagnostics", "getProgramDiagnostics",
+                "getEncodedSyntacticClassifications", "getEncodedSemanticClassifications", "getCompletionsAtPosition",
+                "getCompletionEntryDetails", "getQuickInfoAtPosition", "getNameOrDottedNameSpan",
+                "getBreakpointStatementAtPosition", "getSignatureHelpItems", "getRenameInfo",
+                "findRenameLocations", "getDefinitionAtPosition", "getTypeDefinitionAtPosition",
+                "getReferencesAtPosition", "findReferences", "getDocumentHighlights",
+                "getNavigateToItems", "getNavigationBarItems", "getOutliningSpans",
+                "getTodoComments", "getBraceMatchingAtPosition", "getIndentationAtPosition",
+                "getFormattingEditsForRange", "getFormattingEditsForDocument", "getFormattingEditsAfterKeystroke",
+                "getDocCommentTemplateAtPosition"
+            ]);
+        }
+
+    export function createUnextendedLanguageService(host: LanguageServiceHost,
         documentRegistry: DocumentRegistry = createDocumentRegistry(host.useCaseSensitiveFileNames && host.useCaseSensitiveFileNames(), host.getCurrentDirectory())): LanguageService {
 
         const syntaxTreeCache: SyntaxTreeCache = new SyntaxTreeCache(host);
         let ruleProvider: formatting.RulesProvider;
         let program: Program;
+        let extensionCache: ExtensionCache;
         let lastProjectVersion: string;
 
         const useCaseSensitivefileNames = false;
@@ -3114,11 +3274,13 @@ namespace ts {
                 getCurrentDirectory: () => currentDirectory,
                 fileExists: (fileName): boolean => {
                     // stub missing host functionality
+                    Debug.assert(!!hostCache, "LS CompilerHost may not persist beyond the execution of a synchronize call");
                     Debug.assert(!host.resolveModuleNames || !host.resolveTypeReferenceDirectives);
                     return hostCache.getOrCreateEntry(fileName) !== undefined;
                 },
                 readFile: (fileName): string => {
                     // stub missing host functionality
+                    Debug.assert(!!hostCache, "LS CompilerHost may not persist beyond the execution of a synchronize call");
                     const entry = hostCache.getOrCreateEntry(fileName);
                     return entry && entry.scriptSnapshot.getText(0, entry.scriptSnapshot.getLength());
                 },
@@ -3127,6 +3289,9 @@ namespace ts {
                 },
                 getDirectories: path => {
                     return host.getDirectories ? host.getDirectories(path) : [];
+                },
+                loadExtension: path => {
+                    return host.loadExtension ? host.loadExtension(path) : undefined;
                 }
             };
             if (host.trace) {
@@ -3142,8 +3307,13 @@ namespace ts {
                 };
             }
 
+            const changesInCompilationSettingsAffectExtensions = oldSettings && !deepEqual(oldSettings.extensions, newSettings.extensions);
+            if (!extensionCache || changesInCompilationSettingsAffectExtensions) {
+                extensionCache = createExtensionCache(newSettings, compilerHost);
+            }
+
             const documentRegistryBucketKey = documentRegistry.getKeyForCompilationSettings(newSettings);
-            const newProgram = createProgram(hostCache.getRootFileNames(), newSettings, compilerHost, program);
+            const newProgram = createProgram(hostCache.getRootFileNames(), newSettings, compilerHost, program, extensionCache);
 
             // Release any files we have acquired in the old program but are
             // not part of the new program.
@@ -3304,7 +3474,7 @@ namespace ts {
             return concatenate(semanticDiagnostics, declarationDiagnostics);
         }
 
-        function getCompilerOptionsDiagnostics() {
+        function getProgramDiagnostics() {
             synchronizeHostData();
             return program.getOptionsDiagnostics(cancellationToken).concat(
                    program.getGlobalDiagnostics(cancellationToken));
@@ -8265,7 +8435,8 @@ namespace ts {
             cleanupSemanticCache,
             getSyntacticDiagnostics,
             getSemanticDiagnostics,
-            getCompilerOptionsDiagnostics,
+            getCompilerOptionsDiagnostics: getProgramDiagnostics,
+            getProgramDiagnostics,
             getSyntacticClassifications,
             getSemanticClassifications,
             getEncodedSyntacticClassifications,
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 45c4b284ae744..a73133940bd6e 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -129,6 +129,7 @@ namespace ts {
         getSyntacticDiagnostics(fileName: string): string;
         getSemanticDiagnostics(fileName: string): string;
         getCompilerOptionsDiagnostics(): string;
+        getProgramDiagnostics(): string;
 
         getSyntacticClassifications(fileName: string, start: number, length: number): string;
         getSemanticClassifications(fileName: string, start: number, length: number): string;
@@ -562,11 +563,11 @@ namespace ts {
         }
     }
 
-    export function realizeDiagnostics(diagnostics: Diagnostic[], newLine: string): { message: string; start: number; length: number; category: string; code: number; }[] {
+    export function realizeDiagnostics(diagnostics: Diagnostic[], newLine: string): { message: string; start: number; length: number; category: string; code: number | string; }[] {
         return diagnostics.map(d => realizeDiagnostic(d, newLine));
     }
 
-    function realizeDiagnostic(diagnostic: Diagnostic, newLine: string): { message: string; start: number; length: number; category: string; code: number; } {
+    function realizeDiagnostic(diagnostic: Diagnostic, newLine: string): { message: string; start: number; length: number; category: string; code: number | string; } {
         return {
             message: flattenDiagnosticMessageText(diagnostic.messageText, newLine),
             start: diagnostic.start,
@@ -699,6 +700,15 @@ namespace ts {
                 });
         }
 
+        public getProgramDiagnostics(): string {
+            return this.forwardJSONCall(
+                "getProgramDiagnostics()",
+                () => {
+                    const diagnostics = this.languageService.getProgramDiagnostics();
+                    return this.realizeDiagnostics(diagnostics);
+                });
+        }
+
         /// QUICKINFO
 
         /**
diff --git a/src/services/tsconfig.json b/src/services/tsconfig.json
index cfeb7c2fcd582..7d8d5893212bf 100644
--- a/src/services/tsconfig.json
+++ b/src/services/tsconfig.json
@@ -25,6 +25,7 @@
         "../compiler/declarationEmitter.ts",
         "../compiler/emitter.ts",
         "../compiler/program.ts",
+        "../compiler/extensions.ts",
         "../compiler/commandLineParser.ts",
         "../compiler/diagnosticInformationMap.generated.ts",
         "breakpoints.ts",
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-add-program-diagnostics.js b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-add-program-diagnostics.js
new file mode 100644
index 0000000000000..21b4721f1a6ed
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-add-program-diagnostics.js
@@ -0,0 +1,5 @@
+//// [hello.ts]
+console.log("Hello, world!");/*EOL*/
+
+//// [hello.js]
+console.log("Hello, world!"); /*EOL*/
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.errors.txt b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.errors.txt
new file mode 100644
index 0000000000000..5402510857df8
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.errors.txt
@@ -0,0 +1,10 @@
+/hello-error.ts(1,1): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+/hello-error.ts(1,31): error TS1109: Expression expected.
+
+
+==== /hello-error.ts (2 errors) ====
+    console.log("Hello, error!") -
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+                                  
+!!! error TS1109: Expression expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.js b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.js
new file mode 100644
index 0000000000000..10802294f21ba
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-chain-services.js
@@ -0,0 +1,5 @@
+//// [hello-error.ts]
+console.log("Hello, error!") -
+
+//// [hello-error.js]
+console.log("Hello, error!") - ;
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.errors.txt b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.errors.txt
new file mode 100644
index 0000000000000..c268502890d90
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.errors.txt
@@ -0,0 +1,653 @@
+/atotc.ts(1,1): error TS2304: Cannot find name 'It'.
+/atotc.ts(1,4): error TS1005: ';' expected.
+/atotc.ts(1,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(1,8): error TS1005: ';' expected.
+/atotc.ts(1,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(1,12): error TS1005: ';' expected.
+/atotc.ts(1,12): error TS2304: Cannot find name 'best'.
+/atotc.ts(1,17): error TS1005: ';' expected.
+/atotc.ts(1,17): error TS2304: Cannot find name 'of'.
+/atotc.ts(1,20): error TS1005: ';' expected.
+/atotc.ts(1,20): error TS2304: Cannot find name 'times'.
+/atotc.ts(1,27): error TS2304: Cannot find name 'it'.
+/atotc.ts(1,30): error TS1005: ';' expected.
+/atotc.ts(1,30): error TS2304: Cannot find name 'was'.
+/atotc.ts(1,34): error TS1005: ';' expected.
+/atotc.ts(1,34): error TS2304: Cannot find name 'the'.
+/atotc.ts(1,38): error TS1005: ';' expected.
+/atotc.ts(1,38): error TS2304: Cannot find name 'worst'.
+/atotc.ts(1,44): error TS1005: ';' expected.
+/atotc.ts(1,44): error TS2304: Cannot find name 'of'.
+/atotc.ts(1,47): error TS1005: ';' expected.
+/atotc.ts(1,47): error TS2304: Cannot find name 'times'.
+/atotc.ts(2,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(2,4): error TS1005: ';' expected.
+/atotc.ts(2,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(2,8): error TS1005: ';' expected.
+/atotc.ts(2,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(2,12): error TS1005: ';' expected.
+/atotc.ts(2,12): error TS2304: Cannot find name 'age'.
+/atotc.ts(2,16): error TS1005: ';' expected.
+/atotc.ts(2,16): error TS2304: Cannot find name 'of'.
+/atotc.ts(2,19): error TS1005: ';' expected.
+/atotc.ts(2,19): error TS2304: Cannot find name 'wisdom'.
+/atotc.ts(2,27): error TS2304: Cannot find name 'it'.
+/atotc.ts(2,30): error TS1005: ';' expected.
+/atotc.ts(2,30): error TS2304: Cannot find name 'was'.
+/atotc.ts(2,34): error TS1005: ';' expected.
+/atotc.ts(2,34): error TS2304: Cannot find name 'the'.
+/atotc.ts(2,38): error TS1005: ';' expected.
+/atotc.ts(2,38): error TS2304: Cannot find name 'age'.
+/atotc.ts(2,42): error TS1005: ';' expected.
+/atotc.ts(2,42): error TS2304: Cannot find name 'of'.
+/atotc.ts(2,45): error TS1005: ';' expected.
+/atotc.ts(2,45): error TS2304: Cannot find name 'foolishness'.
+/atotc.ts(3,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(3,4): error TS1005: ';' expected.
+/atotc.ts(3,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(3,8): error TS1005: ';' expected.
+/atotc.ts(3,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(3,12): error TS1005: ';' expected.
+/atotc.ts(3,12): error TS2304: Cannot find name 'epoch'.
+/atotc.ts(3,18): error TS1005: ';' expected.
+/atotc.ts(3,18): error TS2304: Cannot find name 'of'.
+/atotc.ts(3,21): error TS1005: ';' expected.
+/atotc.ts(3,21): error TS2304: Cannot find name 'belief'.
+/atotc.ts(3,29): error TS2304: Cannot find name 'it'.
+/atotc.ts(3,32): error TS1005: ';' expected.
+/atotc.ts(3,32): error TS2304: Cannot find name 'was'.
+/atotc.ts(3,36): error TS1005: ';' expected.
+/atotc.ts(3,36): error TS2304: Cannot find name 'the'.
+/atotc.ts(3,40): error TS1005: ';' expected.
+/atotc.ts(3,40): error TS2304: Cannot find name 'epoch'.
+/atotc.ts(3,46): error TS1005: ';' expected.
+/atotc.ts(3,46): error TS2304: Cannot find name 'of'.
+/atotc.ts(3,49): error TS1005: ';' expected.
+/atotc.ts(3,49): error TS2304: Cannot find name 'incredulity'.
+/atotc.ts(4,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(4,4): error TS1005: ';' expected.
+/atotc.ts(4,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(4,8): error TS1005: ';' expected.
+/atotc.ts(4,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(4,12): error TS1005: ';' expected.
+/atotc.ts(4,12): error TS2304: Cannot find name 'season'.
+/atotc.ts(4,19): error TS1005: ';' expected.
+/atotc.ts(4,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(4,22): error TS1005: ';' expected.
+/atotc.ts(4,22): error TS2304: Cannot find name 'Light'.
+/atotc.ts(4,29): error TS2304: Cannot find name 'it'.
+/atotc.ts(4,32): error TS1005: ';' expected.
+/atotc.ts(4,32): error TS2304: Cannot find name 'was'.
+/atotc.ts(4,36): error TS1005: ';' expected.
+/atotc.ts(4,36): error TS2304: Cannot find name 'the'.
+/atotc.ts(4,40): error TS1005: ';' expected.
+/atotc.ts(4,40): error TS2304: Cannot find name 'season'.
+/atotc.ts(4,47): error TS1005: ';' expected.
+/atotc.ts(4,47): error TS2304: Cannot find name 'of'.
+/atotc.ts(4,50): error TS1005: ';' expected.
+/atotc.ts(4,50): error TS2304: Cannot find name 'Darkness'.
+/atotc.ts(5,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(5,4): error TS1005: ';' expected.
+/atotc.ts(5,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(5,8): error TS1005: ';' expected.
+/atotc.ts(5,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(5,12): error TS1005: ';' expected.
+/atotc.ts(5,12): error TS2304: Cannot find name 'spring'.
+/atotc.ts(5,19): error TS1005: ';' expected.
+/atotc.ts(5,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(5,22): error TS1005: ';' expected.
+/atotc.ts(5,22): error TS2304: Cannot find name 'hope'.
+/atotc.ts(5,28): error TS2304: Cannot find name 'it'.
+/atotc.ts(5,31): error TS1005: ';' expected.
+/atotc.ts(5,31): error TS2304: Cannot find name 'was'.
+/atotc.ts(5,35): error TS1005: ';' expected.
+/atotc.ts(5,35): error TS2304: Cannot find name 'the'.
+/atotc.ts(5,39): error TS1005: ';' expected.
+/atotc.ts(5,39): error TS2304: Cannot find name 'winter'.
+/atotc.ts(5,46): error TS1005: ';' expected.
+/atotc.ts(5,46): error TS2304: Cannot find name 'of'.
+/atotc.ts(5,49): error TS1005: ';' expected.
+/atotc.ts(5,49): error TS2304: Cannot find name 'despair'.
+/atotc.ts(6,1): error TS2304: Cannot find name 'we'.
+/atotc.ts(6,4): error TS1005: ';' expected.
+/atotc.ts(6,4): error TS2304: Cannot find name 'had'.
+/atotc.ts(6,8): error TS1005: ';' expected.
+/atotc.ts(6,8): error TS2304: Cannot find name 'everything'.
+/atotc.ts(6,19): error TS1005: ';' expected.
+/atotc.ts(6,19): error TS2304: Cannot find name 'before'.
+/atotc.ts(6,26): error TS1005: ';' expected.
+/atotc.ts(6,26): error TS2304: Cannot find name 'us'.
+/atotc.ts(6,30): error TS2304: Cannot find name 'we'.
+/atotc.ts(6,33): error TS1005: ';' expected.
+/atotc.ts(6,33): error TS2304: Cannot find name 'had'.
+/atotc.ts(6,37): error TS1005: ';' expected.
+/atotc.ts(6,37): error TS2304: Cannot find name 'nothing'.
+/atotc.ts(6,45): error TS1005: ';' expected.
+/atotc.ts(6,45): error TS2304: Cannot find name 'before'.
+/atotc.ts(6,52): error TS1005: ';' expected.
+/atotc.ts(6,52): error TS2304: Cannot find name 'us'.
+/atotc.ts(7,1): error TS2304: Cannot find name 'we'.
+/atotc.ts(7,4): error TS1005: ';' expected.
+/atotc.ts(7,4): error TS2304: Cannot find name 'were'.
+/atotc.ts(7,9): error TS1005: ';' expected.
+/atotc.ts(7,9): error TS2304: Cannot find name 'all'.
+/atotc.ts(7,13): error TS1005: ';' expected.
+/atotc.ts(7,13): error TS2304: Cannot find name 'going'.
+/atotc.ts(7,19): error TS1005: ';' expected.
+/atotc.ts(7,19): error TS2304: Cannot find name 'direct'.
+/atotc.ts(7,26): error TS1005: ';' expected.
+/atotc.ts(7,26): error TS2304: Cannot find name 'to'.
+/atotc.ts(7,29): error TS1005: ';' expected.
+/atotc.ts(7,29): error TS2304: Cannot find name 'Heaven'.
+/atotc.ts(7,37): error TS2304: Cannot find name 'we'.
+/atotc.ts(7,40): error TS1005: ';' expected.
+/atotc.ts(7,40): error TS2304: Cannot find name 'were'.
+/atotc.ts(7,45): error TS1005: ';' expected.
+/atotc.ts(7,45): error TS2304: Cannot find name 'all'.
+/atotc.ts(7,49): error TS1005: ';' expected.
+/atotc.ts(7,49): error TS2304: Cannot find name 'going'.
+/atotc.ts(7,55): error TS1005: ';' expected.
+/atotc.ts(7,55): error TS2304: Cannot find name 'direct'.
+/atotc.ts(8,1): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,5): error TS1005: ';' expected.
+/atotc.ts(8,5): error TS2304: Cannot find name 'other'.
+/atotc.ts(8,11): error TS1005: ';' expected.
+/atotc.ts(8,11): error TS2304: Cannot find name 'way'.
+/atotc.ts(8,19): error TS2304: Cannot find name 'short'.
+/atotc.ts(8,26): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,30): error TS1005: ';' expected.
+/atotc.ts(8,30): error TS2304: Cannot find name 'period'.
+/atotc.ts(8,37): error TS1005: ';' expected.
+/atotc.ts(8,37): error TS2304: Cannot find name 'was'.
+/atotc.ts(8,41): error TS1005: ';' expected.
+/atotc.ts(8,41): error TS2304: Cannot find name 'so'.
+/atotc.ts(8,44): error TS1005: ';' expected.
+/atotc.ts(8,44): error TS2304: Cannot find name 'far'.
+/atotc.ts(8,48): error TS1005: ';' expected.
+/atotc.ts(8,48): error TS2304: Cannot find name 'like'.
+/atotc.ts(8,53): error TS1005: ';' expected.
+/atotc.ts(8,53): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,57): error TS1005: ';' expected.
+/atotc.ts(8,57): error TS2304: Cannot find name 'present'.
+/atotc.ts(9,1): error TS2304: Cannot find name 'period'.
+/atotc.ts(9,9): error TS2304: Cannot find name 'that'.
+/atotc.ts(9,14): error TS1005: ';' expected.
+/atotc.ts(9,14): error TS2304: Cannot find name 'some'.
+/atotc.ts(9,19): error TS1005: ';' expected.
+/atotc.ts(9,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(9,22): error TS1005: ';' expected.
+/atotc.ts(9,22): error TS2304: Cannot find name 'its'.
+/atotc.ts(9,26): error TS1005: ';' expected.
+/atotc.ts(9,26): error TS2304: Cannot find name 'noisiest'.
+/atotc.ts(9,35): error TS1005: ';' expected.
+/atotc.ts(9,35): error TS2304: Cannot find name 'authorities'.
+/atotc.ts(9,47): error TS1005: ';' expected.
+/atotc.ts(9,47): error TS2304: Cannot find name 'insisted'.
+/atotc.ts(9,56): error TS1005: ';' expected.
+/atotc.ts(9,56): error TS2304: Cannot find name 'on'.
+/atotc.ts(9,59): error TS1005: ';' expected.
+/atotc.ts(9,59): error TS2304: Cannot find name 'its'.
+/atotc.ts(10,1): error TS2304: Cannot find name 'being'.
+/atotc.ts(10,7): error TS1005: ';' expected.
+/atotc.ts(10,7): error TS2304: Cannot find name 'received'.
+/atotc.ts(10,17): error TS1109: Expression expected.
+/atotc.ts(10,21): error TS1005: '(' expected.
+/atotc.ts(10,21): error TS2304: Cannot find name 'good'.
+/atotc.ts(10,26): error TS1005: ';' expected.
+/atotc.ts(10,26): error TS2304: Cannot find name 'or'.
+/atotc.ts(10,29): error TS1005: ';' expected.
+/atotc.ts(10,33): error TS1005: '(' expected.
+/atotc.ts(10,33): error TS2304: Cannot find name 'evil'.
+/atotc.ts(10,33): error TS2406: Invalid left-hand side in 'for...in' statement.
+/atotc.ts(10,39): error TS1109: Expression expected.
+/atotc.ts(10,42): error TS2304: Cannot find name 'the'.
+/atotc.ts(10,46): error TS1005: ')' expected.
+/atotc.ts(10,46): error TS2304: Cannot find name 'superlative'.
+/atotc.ts(10,58): error TS1005: ';' expected.
+/atotc.ts(10,58): error TS2304: Cannot find name 'degree'.
+/atotc.ts(11,1): error TS2304: Cannot find name 'of'.
+/atotc.ts(11,4): error TS1005: ';' expected.
+/atotc.ts(11,4): error TS2304: Cannot find name 'comparison'.
+/atotc.ts(11,15): error TS1005: ';' expected.
+/atotc.ts(11,15): error TS2304: Cannot find name 'only'.
+/atotc.ts(11,20): error TS1003: Identifier expected.
+
+
+==== /atotc.ts (213 errors) ====
+    It was the best of times, it was the worst of times,
+    ~~
+!!! error TS2304: Cannot find name 'It'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~
+!!! error TS1005: ';' expected.
+               ~~~~
+!!! error TS2304: Cannot find name 'best'.
+                    ~~
+!!! error TS1005: ';' expected.
+                    ~~
+!!! error TS2304: Cannot find name 'of'.
+                       ~~~~~
+!!! error TS1005: ';' expected.
+                       ~~~~~
+!!! error TS2304: Cannot find name 'times'.
+                              ~~
+!!! error TS2304: Cannot find name 'it'.
+                                 ~~~
+!!! error TS1005: ';' expected.
+                                 ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                     ~~~
+!!! error TS1005: ';' expected.
+                                     ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                         ~~~~~
+!!! error TS1005: ';' expected.
+                                         ~~~~~
+!!! error TS2304: Cannot find name 'worst'.
+                                               ~~
+!!! error TS1005: ';' expected.
+                                               ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                  ~~~~~
+!!! error TS1005: ';' expected.
+                                                  ~~~~~
+!!! error TS2304: Cannot find name 'times'.
+    it was the age of wisdom, it was the age of foolishness,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~
+!!! error TS1005: ';' expected.
+               ~~~
+!!! error TS2304: Cannot find name 'age'.
+                   ~~
+!!! error TS1005: ';' expected.
+                   ~~
+!!! error TS2304: Cannot find name 'of'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'wisdom'.
+                              ~~
+!!! error TS2304: Cannot find name 'it'.
+                                 ~~~
+!!! error TS1005: ';' expected.
+                                 ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                     ~~~
+!!! error TS1005: ';' expected.
+                                     ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                         ~~~
+!!! error TS1005: ';' expected.
+                                         ~~~
+!!! error TS2304: Cannot find name 'age'.
+                                             ~~
+!!! error TS1005: ';' expected.
+                                             ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'foolishness'.
+    it was the epoch of belief, it was the epoch of incredulity,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~
+!!! error TS2304: Cannot find name 'epoch'.
+                     ~~
+!!! error TS1005: ';' expected.
+                     ~~
+!!! error TS2304: Cannot find name 'of'.
+                        ~~~~~~
+!!! error TS1005: ';' expected.
+                        ~~~~~~
+!!! error TS2304: Cannot find name 'belief'.
+                                ~~
+!!! error TS2304: Cannot find name 'it'.
+                                   ~~~
+!!! error TS1005: ';' expected.
+                                   ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                       ~~~
+!!! error TS1005: ';' expected.
+                                       ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                           ~~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~~
+!!! error TS2304: Cannot find name 'epoch'.
+                                                 ~~
+!!! error TS1005: ';' expected.
+                                                 ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                    ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'incredulity'.
+    it was the season of Light, it was the season of Darkness,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~~
+!!! error TS2304: Cannot find name 'season'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~~~
+!!! error TS1005: ';' expected.
+                         ~~~~~
+!!! error TS2304: Cannot find name 'Light'.
+                                ~~
+!!! error TS2304: Cannot find name 'it'.
+                                   ~~~
+!!! error TS1005: ';' expected.
+                                   ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                       ~~~
+!!! error TS1005: ';' expected.
+                                       ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                           ~~~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~~~
+!!! error TS2304: Cannot find name 'season'.
+                                                  ~~
+!!! error TS1005: ';' expected.
+                                                  ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                     ~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                     ~~~~~~~~
+!!! error TS2304: Cannot find name 'Darkness'.
+    it was the spring of hope, it was the winter of despair,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~~
+!!! error TS2304: Cannot find name 'spring'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~~
+!!! error TS1005: ';' expected.
+                         ~~~~
+!!! error TS2304: Cannot find name 'hope'.
+                               ~~
+!!! error TS2304: Cannot find name 'it'.
+                                  ~~~
+!!! error TS1005: ';' expected.
+                                  ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                      ~~~
+!!! error TS1005: ';' expected.
+                                      ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                          ~~~~~~
+!!! error TS1005: ';' expected.
+                                          ~~~~~~
+!!! error TS2304: Cannot find name 'winter'.
+                                                 ~~
+!!! error TS1005: ';' expected.
+                                                 ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                    ~~~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~~~
+!!! error TS2304: Cannot find name 'despair'.
+    we had everything before us, we had nothing before us,
+    ~~
+!!! error TS2304: Cannot find name 'we'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'had'.
+           ~~~~~~~~~~
+!!! error TS1005: ';' expected.
+           ~~~~~~~~~~
+!!! error TS2304: Cannot find name 'everything'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'before'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'us'.
+                                 ~~
+!!! error TS2304: Cannot find name 'we'.
+                                    ~~~
+!!! error TS1005: ';' expected.
+                                    ~~~
+!!! error TS2304: Cannot find name 'had'.
+                                        ~~~~~~~
+!!! error TS1005: ';' expected.
+                                        ~~~~~~~
+!!! error TS2304: Cannot find name 'nothing'.
+                                                ~~~~~~
+!!! error TS1005: ';' expected.
+                                                ~~~~~~
+!!! error TS2304: Cannot find name 'before'.
+                                                       ~~
+!!! error TS1005: ';' expected.
+                                                       ~~
+!!! error TS2304: Cannot find name 'us'.
+    we were all going direct to Heaven, we were all going direct
+    ~~
+!!! error TS2304: Cannot find name 'we'.
+       ~~~~
+!!! error TS1005: ';' expected.
+       ~~~~
+!!! error TS2304: Cannot find name 'were'.
+            ~~~
+!!! error TS1005: ';' expected.
+            ~~~
+!!! error TS2304: Cannot find name 'all'.
+                ~~~~~
+!!! error TS1005: ';' expected.
+                ~~~~~
+!!! error TS2304: Cannot find name 'going'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'direct'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'to'.
+                                ~~~~~~
+!!! error TS1005: ';' expected.
+                                ~~~~~~
+!!! error TS2304: Cannot find name 'Heaven'.
+                                        ~~
+!!! error TS2304: Cannot find name 'we'.
+                                           ~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~
+!!! error TS2304: Cannot find name 'were'.
+                                                ~~~
+!!! error TS1005: ';' expected.
+                                                ~~~
+!!! error TS2304: Cannot find name 'all'.
+                                                    ~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~
+!!! error TS2304: Cannot find name 'going'.
+                                                          ~~~~~~
+!!! error TS1005: ';' expected.
+                                                          ~~~~~~
+!!! error TS2304: Cannot find name 'direct'.
+    the other way--in short, the period was so far like the present
+    ~~~
+!!! error TS2304: Cannot find name 'the'.
+        ~~~~~
+!!! error TS1005: ';' expected.
+        ~~~~~
+!!! error TS2304: Cannot find name 'other'.
+              ~~~
+!!! error TS1005: ';' expected.
+              ~~~
+!!! error TS2304: Cannot find name 'way'.
+                      ~~~~~
+!!! error TS2304: Cannot find name 'short'.
+                             ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                 ~~~~~~
+!!! error TS1005: ';' expected.
+                                 ~~~~~~
+!!! error TS2304: Cannot find name 'period'.
+                                        ~~~
+!!! error TS1005: ';' expected.
+                                        ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                            ~~
+!!! error TS1005: ';' expected.
+                                            ~~
+!!! error TS2304: Cannot find name 'so'.
+                                               ~~~
+!!! error TS1005: ';' expected.
+                                               ~~~
+!!! error TS2304: Cannot find name 'far'.
+                                                   ~~~~
+!!! error TS1005: ';' expected.
+                                                   ~~~~
+!!! error TS2304: Cannot find name 'like'.
+                                                        ~~~
+!!! error TS1005: ';' expected.
+                                                        ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                                            ~~~~~~~
+!!! error TS1005: ';' expected.
+                                                            ~~~~~~~
+!!! error TS2304: Cannot find name 'present'.
+    period, that some of its noisiest authorities insisted on its
+    ~~~~~~
+!!! error TS2304: Cannot find name 'period'.
+            ~~~~
+!!! error TS2304: Cannot find name 'that'.
+                 ~~~~
+!!! error TS1005: ';' expected.
+                 ~~~~
+!!! error TS2304: Cannot find name 'some'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~
+!!! error TS1005: ';' expected.
+                         ~~~
+!!! error TS2304: Cannot find name 'its'.
+                             ~~~~~~~~
+!!! error TS1005: ';' expected.
+                             ~~~~~~~~
+!!! error TS2304: Cannot find name 'noisiest'.
+                                      ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                      ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'authorities'.
+                                                  ~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                  ~~~~~~~~
+!!! error TS2304: Cannot find name 'insisted'.
+                                                           ~~
+!!! error TS1005: ';' expected.
+                                                           ~~
+!!! error TS2304: Cannot find name 'on'.
+                                                              ~~~
+!!! error TS1005: ';' expected.
+                                                              ~~~
+!!! error TS2304: Cannot find name 'its'.
+    being received, for good or for evil, in the superlative degree
+    ~~~~~
+!!! error TS2304: Cannot find name 'being'.
+          ~~~~~~~~
+!!! error TS1005: ';' expected.
+          ~~~~~~~~
+!!! error TS2304: Cannot find name 'received'.
+                    ~~~
+!!! error TS1109: Expression expected.
+                        ~~~~
+!!! error TS1005: '(' expected.
+                        ~~~~
+!!! error TS2304: Cannot find name 'good'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'or'.
+                                ~~~
+!!! error TS1005: ';' expected.
+                                    ~~~~
+!!! error TS1005: '(' expected.
+                                    ~~~~
+!!! error TS2304: Cannot find name 'evil'.
+                                    ~~~~~
+!!! error TS2406: Invalid left-hand side in 'for...in' statement.
+                                          ~~
+!!! error TS1109: Expression expected.
+                                             ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                                 ~~~~~~~~~~~
+!!! error TS1005: ')' expected.
+                                                 ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'superlative'.
+                                                             ~~~~~~
+!!! error TS1005: ';' expected.
+                                                             ~~~~~~
+!!! error TS2304: Cannot find name 'degree'.
+    of comparison only.
+    ~~
+!!! error TS2304: Cannot find name 'of'.
+       ~~~~~~~~~~
+!!! error TS1005: ';' expected.
+       ~~~~~~~~~~
+!!! error TS2304: Cannot find name 'comparison'.
+                  ~~~~
+!!! error TS1005: ';' expected.
+                  ~~~~
+!!! error TS2304: Cannot find name 'only'.
+                       
+!!! error TS1003: Identifier expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.js b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.js
new file mode 100644
index 0000000000000..30c1de3575183
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-all-functions.js
@@ -0,0 +1,117 @@
+//// [atotc.ts]
+It was the best of times, it was the worst of times,
+it was the age of wisdom, it was the age of foolishness,
+it was the epoch of belief, it was the epoch of incredulity,
+it was the season of Light, it was the season of Darkness,
+it was the spring of hope, it was the winter of despair,
+we had everything before us, we had nothing before us,
+we were all going direct to Heaven, we were all going direct
+the other way--in short, the period was so far like the present
+period, that some of its noisiest authorities insisted on its
+being received, for good or for evil, in the superlative degree
+of comparison only.
+
+//// [atotc.js]
+It;
+was;
+the;
+best;
+of;
+times, it;
+was;
+the;
+worst;
+of;
+times,
+    it;
+was;
+the;
+age;
+of;
+wisdom, it;
+was;
+the;
+age;
+of;
+foolishness,
+    it;
+was;
+the;
+epoch;
+of;
+belief, it;
+was;
+the;
+epoch;
+of;
+incredulity,
+    it;
+was;
+the;
+season;
+of;
+Light, it;
+was;
+the;
+season;
+of;
+Darkness,
+    it;
+was;
+the;
+spring;
+of;
+hope, it;
+was;
+the;
+winter;
+of;
+despair,
+    we;
+had;
+everything;
+before;
+us, we;
+had;
+nothing;
+before;
+us,
+    we;
+were;
+all;
+going;
+direct;
+to;
+Heaven, we;
+were;
+all;
+going;
+direct;
+the;
+other;
+way-- in short, the;
+period;
+was;
+so;
+far;
+like;
+the;
+present;
+period, that;
+some;
+of;
+its;
+noisiest;
+authorities;
+insisted;
+on;
+its;
+being;
+received, ;
+for (good; or; )
+    for (evil,  in the)
+        superlative;
+degree;
+of;
+comparison;
+only.;
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.errors.txt b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.errors.txt
new file mode 100644
index 0000000000000..5402510857df8
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.errors.txt
@@ -0,0 +1,10 @@
+/hello-error.ts(1,1): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+/hello-error.ts(1,31): error TS1109: Expression expected.
+
+
+==== /hello-error.ts (2 errors) ====
+    console.log("Hello, error!") -
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+                                  
+!!! error TS1109: Expression expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.js b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.js
new file mode 100644
index 0000000000000..10802294f21ba
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-filter-passthru.js
@@ -0,0 +1,5 @@
+//// [hello-error.ts]
+console.log("Hello, error!") -
+
+//// [hello-error.js]
+console.log("Hello, error!") - ;
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.errors.txt b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.errors.txt
new file mode 100644
index 0000000000000..c268502890d90
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.errors.txt
@@ -0,0 +1,653 @@
+/atotc.ts(1,1): error TS2304: Cannot find name 'It'.
+/atotc.ts(1,4): error TS1005: ';' expected.
+/atotc.ts(1,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(1,8): error TS1005: ';' expected.
+/atotc.ts(1,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(1,12): error TS1005: ';' expected.
+/atotc.ts(1,12): error TS2304: Cannot find name 'best'.
+/atotc.ts(1,17): error TS1005: ';' expected.
+/atotc.ts(1,17): error TS2304: Cannot find name 'of'.
+/atotc.ts(1,20): error TS1005: ';' expected.
+/atotc.ts(1,20): error TS2304: Cannot find name 'times'.
+/atotc.ts(1,27): error TS2304: Cannot find name 'it'.
+/atotc.ts(1,30): error TS1005: ';' expected.
+/atotc.ts(1,30): error TS2304: Cannot find name 'was'.
+/atotc.ts(1,34): error TS1005: ';' expected.
+/atotc.ts(1,34): error TS2304: Cannot find name 'the'.
+/atotc.ts(1,38): error TS1005: ';' expected.
+/atotc.ts(1,38): error TS2304: Cannot find name 'worst'.
+/atotc.ts(1,44): error TS1005: ';' expected.
+/atotc.ts(1,44): error TS2304: Cannot find name 'of'.
+/atotc.ts(1,47): error TS1005: ';' expected.
+/atotc.ts(1,47): error TS2304: Cannot find name 'times'.
+/atotc.ts(2,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(2,4): error TS1005: ';' expected.
+/atotc.ts(2,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(2,8): error TS1005: ';' expected.
+/atotc.ts(2,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(2,12): error TS1005: ';' expected.
+/atotc.ts(2,12): error TS2304: Cannot find name 'age'.
+/atotc.ts(2,16): error TS1005: ';' expected.
+/atotc.ts(2,16): error TS2304: Cannot find name 'of'.
+/atotc.ts(2,19): error TS1005: ';' expected.
+/atotc.ts(2,19): error TS2304: Cannot find name 'wisdom'.
+/atotc.ts(2,27): error TS2304: Cannot find name 'it'.
+/atotc.ts(2,30): error TS1005: ';' expected.
+/atotc.ts(2,30): error TS2304: Cannot find name 'was'.
+/atotc.ts(2,34): error TS1005: ';' expected.
+/atotc.ts(2,34): error TS2304: Cannot find name 'the'.
+/atotc.ts(2,38): error TS1005: ';' expected.
+/atotc.ts(2,38): error TS2304: Cannot find name 'age'.
+/atotc.ts(2,42): error TS1005: ';' expected.
+/atotc.ts(2,42): error TS2304: Cannot find name 'of'.
+/atotc.ts(2,45): error TS1005: ';' expected.
+/atotc.ts(2,45): error TS2304: Cannot find name 'foolishness'.
+/atotc.ts(3,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(3,4): error TS1005: ';' expected.
+/atotc.ts(3,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(3,8): error TS1005: ';' expected.
+/atotc.ts(3,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(3,12): error TS1005: ';' expected.
+/atotc.ts(3,12): error TS2304: Cannot find name 'epoch'.
+/atotc.ts(3,18): error TS1005: ';' expected.
+/atotc.ts(3,18): error TS2304: Cannot find name 'of'.
+/atotc.ts(3,21): error TS1005: ';' expected.
+/atotc.ts(3,21): error TS2304: Cannot find name 'belief'.
+/atotc.ts(3,29): error TS2304: Cannot find name 'it'.
+/atotc.ts(3,32): error TS1005: ';' expected.
+/atotc.ts(3,32): error TS2304: Cannot find name 'was'.
+/atotc.ts(3,36): error TS1005: ';' expected.
+/atotc.ts(3,36): error TS2304: Cannot find name 'the'.
+/atotc.ts(3,40): error TS1005: ';' expected.
+/atotc.ts(3,40): error TS2304: Cannot find name 'epoch'.
+/atotc.ts(3,46): error TS1005: ';' expected.
+/atotc.ts(3,46): error TS2304: Cannot find name 'of'.
+/atotc.ts(3,49): error TS1005: ';' expected.
+/atotc.ts(3,49): error TS2304: Cannot find name 'incredulity'.
+/atotc.ts(4,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(4,4): error TS1005: ';' expected.
+/atotc.ts(4,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(4,8): error TS1005: ';' expected.
+/atotc.ts(4,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(4,12): error TS1005: ';' expected.
+/atotc.ts(4,12): error TS2304: Cannot find name 'season'.
+/atotc.ts(4,19): error TS1005: ';' expected.
+/atotc.ts(4,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(4,22): error TS1005: ';' expected.
+/atotc.ts(4,22): error TS2304: Cannot find name 'Light'.
+/atotc.ts(4,29): error TS2304: Cannot find name 'it'.
+/atotc.ts(4,32): error TS1005: ';' expected.
+/atotc.ts(4,32): error TS2304: Cannot find name 'was'.
+/atotc.ts(4,36): error TS1005: ';' expected.
+/atotc.ts(4,36): error TS2304: Cannot find name 'the'.
+/atotc.ts(4,40): error TS1005: ';' expected.
+/atotc.ts(4,40): error TS2304: Cannot find name 'season'.
+/atotc.ts(4,47): error TS1005: ';' expected.
+/atotc.ts(4,47): error TS2304: Cannot find name 'of'.
+/atotc.ts(4,50): error TS1005: ';' expected.
+/atotc.ts(4,50): error TS2304: Cannot find name 'Darkness'.
+/atotc.ts(5,1): error TS2304: Cannot find name 'it'.
+/atotc.ts(5,4): error TS1005: ';' expected.
+/atotc.ts(5,4): error TS2304: Cannot find name 'was'.
+/atotc.ts(5,8): error TS1005: ';' expected.
+/atotc.ts(5,8): error TS2304: Cannot find name 'the'.
+/atotc.ts(5,12): error TS1005: ';' expected.
+/atotc.ts(5,12): error TS2304: Cannot find name 'spring'.
+/atotc.ts(5,19): error TS1005: ';' expected.
+/atotc.ts(5,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(5,22): error TS1005: ';' expected.
+/atotc.ts(5,22): error TS2304: Cannot find name 'hope'.
+/atotc.ts(5,28): error TS2304: Cannot find name 'it'.
+/atotc.ts(5,31): error TS1005: ';' expected.
+/atotc.ts(5,31): error TS2304: Cannot find name 'was'.
+/atotc.ts(5,35): error TS1005: ';' expected.
+/atotc.ts(5,35): error TS2304: Cannot find name 'the'.
+/atotc.ts(5,39): error TS1005: ';' expected.
+/atotc.ts(5,39): error TS2304: Cannot find name 'winter'.
+/atotc.ts(5,46): error TS1005: ';' expected.
+/atotc.ts(5,46): error TS2304: Cannot find name 'of'.
+/atotc.ts(5,49): error TS1005: ';' expected.
+/atotc.ts(5,49): error TS2304: Cannot find name 'despair'.
+/atotc.ts(6,1): error TS2304: Cannot find name 'we'.
+/atotc.ts(6,4): error TS1005: ';' expected.
+/atotc.ts(6,4): error TS2304: Cannot find name 'had'.
+/atotc.ts(6,8): error TS1005: ';' expected.
+/atotc.ts(6,8): error TS2304: Cannot find name 'everything'.
+/atotc.ts(6,19): error TS1005: ';' expected.
+/atotc.ts(6,19): error TS2304: Cannot find name 'before'.
+/atotc.ts(6,26): error TS1005: ';' expected.
+/atotc.ts(6,26): error TS2304: Cannot find name 'us'.
+/atotc.ts(6,30): error TS2304: Cannot find name 'we'.
+/atotc.ts(6,33): error TS1005: ';' expected.
+/atotc.ts(6,33): error TS2304: Cannot find name 'had'.
+/atotc.ts(6,37): error TS1005: ';' expected.
+/atotc.ts(6,37): error TS2304: Cannot find name 'nothing'.
+/atotc.ts(6,45): error TS1005: ';' expected.
+/atotc.ts(6,45): error TS2304: Cannot find name 'before'.
+/atotc.ts(6,52): error TS1005: ';' expected.
+/atotc.ts(6,52): error TS2304: Cannot find name 'us'.
+/atotc.ts(7,1): error TS2304: Cannot find name 'we'.
+/atotc.ts(7,4): error TS1005: ';' expected.
+/atotc.ts(7,4): error TS2304: Cannot find name 'were'.
+/atotc.ts(7,9): error TS1005: ';' expected.
+/atotc.ts(7,9): error TS2304: Cannot find name 'all'.
+/atotc.ts(7,13): error TS1005: ';' expected.
+/atotc.ts(7,13): error TS2304: Cannot find name 'going'.
+/atotc.ts(7,19): error TS1005: ';' expected.
+/atotc.ts(7,19): error TS2304: Cannot find name 'direct'.
+/atotc.ts(7,26): error TS1005: ';' expected.
+/atotc.ts(7,26): error TS2304: Cannot find name 'to'.
+/atotc.ts(7,29): error TS1005: ';' expected.
+/atotc.ts(7,29): error TS2304: Cannot find name 'Heaven'.
+/atotc.ts(7,37): error TS2304: Cannot find name 'we'.
+/atotc.ts(7,40): error TS1005: ';' expected.
+/atotc.ts(7,40): error TS2304: Cannot find name 'were'.
+/atotc.ts(7,45): error TS1005: ';' expected.
+/atotc.ts(7,45): error TS2304: Cannot find name 'all'.
+/atotc.ts(7,49): error TS1005: ';' expected.
+/atotc.ts(7,49): error TS2304: Cannot find name 'going'.
+/atotc.ts(7,55): error TS1005: ';' expected.
+/atotc.ts(7,55): error TS2304: Cannot find name 'direct'.
+/atotc.ts(8,1): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,5): error TS1005: ';' expected.
+/atotc.ts(8,5): error TS2304: Cannot find name 'other'.
+/atotc.ts(8,11): error TS1005: ';' expected.
+/atotc.ts(8,11): error TS2304: Cannot find name 'way'.
+/atotc.ts(8,19): error TS2304: Cannot find name 'short'.
+/atotc.ts(8,26): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,30): error TS1005: ';' expected.
+/atotc.ts(8,30): error TS2304: Cannot find name 'period'.
+/atotc.ts(8,37): error TS1005: ';' expected.
+/atotc.ts(8,37): error TS2304: Cannot find name 'was'.
+/atotc.ts(8,41): error TS1005: ';' expected.
+/atotc.ts(8,41): error TS2304: Cannot find name 'so'.
+/atotc.ts(8,44): error TS1005: ';' expected.
+/atotc.ts(8,44): error TS2304: Cannot find name 'far'.
+/atotc.ts(8,48): error TS1005: ';' expected.
+/atotc.ts(8,48): error TS2304: Cannot find name 'like'.
+/atotc.ts(8,53): error TS1005: ';' expected.
+/atotc.ts(8,53): error TS2304: Cannot find name 'the'.
+/atotc.ts(8,57): error TS1005: ';' expected.
+/atotc.ts(8,57): error TS2304: Cannot find name 'present'.
+/atotc.ts(9,1): error TS2304: Cannot find name 'period'.
+/atotc.ts(9,9): error TS2304: Cannot find name 'that'.
+/atotc.ts(9,14): error TS1005: ';' expected.
+/atotc.ts(9,14): error TS2304: Cannot find name 'some'.
+/atotc.ts(9,19): error TS1005: ';' expected.
+/atotc.ts(9,19): error TS2304: Cannot find name 'of'.
+/atotc.ts(9,22): error TS1005: ';' expected.
+/atotc.ts(9,22): error TS2304: Cannot find name 'its'.
+/atotc.ts(9,26): error TS1005: ';' expected.
+/atotc.ts(9,26): error TS2304: Cannot find name 'noisiest'.
+/atotc.ts(9,35): error TS1005: ';' expected.
+/atotc.ts(9,35): error TS2304: Cannot find name 'authorities'.
+/atotc.ts(9,47): error TS1005: ';' expected.
+/atotc.ts(9,47): error TS2304: Cannot find name 'insisted'.
+/atotc.ts(9,56): error TS1005: ';' expected.
+/atotc.ts(9,56): error TS2304: Cannot find name 'on'.
+/atotc.ts(9,59): error TS1005: ';' expected.
+/atotc.ts(9,59): error TS2304: Cannot find name 'its'.
+/atotc.ts(10,1): error TS2304: Cannot find name 'being'.
+/atotc.ts(10,7): error TS1005: ';' expected.
+/atotc.ts(10,7): error TS2304: Cannot find name 'received'.
+/atotc.ts(10,17): error TS1109: Expression expected.
+/atotc.ts(10,21): error TS1005: '(' expected.
+/atotc.ts(10,21): error TS2304: Cannot find name 'good'.
+/atotc.ts(10,26): error TS1005: ';' expected.
+/atotc.ts(10,26): error TS2304: Cannot find name 'or'.
+/atotc.ts(10,29): error TS1005: ';' expected.
+/atotc.ts(10,33): error TS1005: '(' expected.
+/atotc.ts(10,33): error TS2304: Cannot find name 'evil'.
+/atotc.ts(10,33): error TS2406: Invalid left-hand side in 'for...in' statement.
+/atotc.ts(10,39): error TS1109: Expression expected.
+/atotc.ts(10,42): error TS2304: Cannot find name 'the'.
+/atotc.ts(10,46): error TS1005: ')' expected.
+/atotc.ts(10,46): error TS2304: Cannot find name 'superlative'.
+/atotc.ts(10,58): error TS1005: ';' expected.
+/atotc.ts(10,58): error TS2304: Cannot find name 'degree'.
+/atotc.ts(11,1): error TS2304: Cannot find name 'of'.
+/atotc.ts(11,4): error TS1005: ';' expected.
+/atotc.ts(11,4): error TS2304: Cannot find name 'comparison'.
+/atotc.ts(11,15): error TS1005: ';' expected.
+/atotc.ts(11,15): error TS2304: Cannot find name 'only'.
+/atotc.ts(11,20): error TS1003: Identifier expected.
+
+
+==== /atotc.ts (213 errors) ====
+    It was the best of times, it was the worst of times,
+    ~~
+!!! error TS2304: Cannot find name 'It'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~
+!!! error TS1005: ';' expected.
+               ~~~~
+!!! error TS2304: Cannot find name 'best'.
+                    ~~
+!!! error TS1005: ';' expected.
+                    ~~
+!!! error TS2304: Cannot find name 'of'.
+                       ~~~~~
+!!! error TS1005: ';' expected.
+                       ~~~~~
+!!! error TS2304: Cannot find name 'times'.
+                              ~~
+!!! error TS2304: Cannot find name 'it'.
+                                 ~~~
+!!! error TS1005: ';' expected.
+                                 ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                     ~~~
+!!! error TS1005: ';' expected.
+                                     ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                         ~~~~~
+!!! error TS1005: ';' expected.
+                                         ~~~~~
+!!! error TS2304: Cannot find name 'worst'.
+                                               ~~
+!!! error TS1005: ';' expected.
+                                               ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                  ~~~~~
+!!! error TS1005: ';' expected.
+                                                  ~~~~~
+!!! error TS2304: Cannot find name 'times'.
+    it was the age of wisdom, it was the age of foolishness,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~
+!!! error TS1005: ';' expected.
+               ~~~
+!!! error TS2304: Cannot find name 'age'.
+                   ~~
+!!! error TS1005: ';' expected.
+                   ~~
+!!! error TS2304: Cannot find name 'of'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'wisdom'.
+                              ~~
+!!! error TS2304: Cannot find name 'it'.
+                                 ~~~
+!!! error TS1005: ';' expected.
+                                 ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                     ~~~
+!!! error TS1005: ';' expected.
+                                     ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                         ~~~
+!!! error TS1005: ';' expected.
+                                         ~~~
+!!! error TS2304: Cannot find name 'age'.
+                                             ~~
+!!! error TS1005: ';' expected.
+                                             ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'foolishness'.
+    it was the epoch of belief, it was the epoch of incredulity,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~
+!!! error TS2304: Cannot find name 'epoch'.
+                     ~~
+!!! error TS1005: ';' expected.
+                     ~~
+!!! error TS2304: Cannot find name 'of'.
+                        ~~~~~~
+!!! error TS1005: ';' expected.
+                        ~~~~~~
+!!! error TS2304: Cannot find name 'belief'.
+                                ~~
+!!! error TS2304: Cannot find name 'it'.
+                                   ~~~
+!!! error TS1005: ';' expected.
+                                   ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                       ~~~
+!!! error TS1005: ';' expected.
+                                       ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                           ~~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~~
+!!! error TS2304: Cannot find name 'epoch'.
+                                                 ~~
+!!! error TS1005: ';' expected.
+                                                 ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                    ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'incredulity'.
+    it was the season of Light, it was the season of Darkness,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~~
+!!! error TS2304: Cannot find name 'season'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~~~
+!!! error TS1005: ';' expected.
+                         ~~~~~
+!!! error TS2304: Cannot find name 'Light'.
+                                ~~
+!!! error TS2304: Cannot find name 'it'.
+                                   ~~~
+!!! error TS1005: ';' expected.
+                                   ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                       ~~~
+!!! error TS1005: ';' expected.
+                                       ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                           ~~~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~~~
+!!! error TS2304: Cannot find name 'season'.
+                                                  ~~
+!!! error TS1005: ';' expected.
+                                                  ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                     ~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                     ~~~~~~~~
+!!! error TS2304: Cannot find name 'Darkness'.
+    it was the spring of hope, it was the winter of despair,
+    ~~
+!!! error TS2304: Cannot find name 'it'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'was'.
+           ~~~
+!!! error TS1005: ';' expected.
+           ~~~
+!!! error TS2304: Cannot find name 'the'.
+               ~~~~~~
+!!! error TS1005: ';' expected.
+               ~~~~~~
+!!! error TS2304: Cannot find name 'spring'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~~
+!!! error TS1005: ';' expected.
+                         ~~~~
+!!! error TS2304: Cannot find name 'hope'.
+                               ~~
+!!! error TS2304: Cannot find name 'it'.
+                                  ~~~
+!!! error TS1005: ';' expected.
+                                  ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                      ~~~
+!!! error TS1005: ';' expected.
+                                      ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                          ~~~~~~
+!!! error TS1005: ';' expected.
+                                          ~~~~~~
+!!! error TS2304: Cannot find name 'winter'.
+                                                 ~~
+!!! error TS1005: ';' expected.
+                                                 ~~
+!!! error TS2304: Cannot find name 'of'.
+                                                    ~~~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~~~
+!!! error TS2304: Cannot find name 'despair'.
+    we had everything before us, we had nothing before us,
+    ~~
+!!! error TS2304: Cannot find name 'we'.
+       ~~~
+!!! error TS1005: ';' expected.
+       ~~~
+!!! error TS2304: Cannot find name 'had'.
+           ~~~~~~~~~~
+!!! error TS1005: ';' expected.
+           ~~~~~~~~~~
+!!! error TS2304: Cannot find name 'everything'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'before'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'us'.
+                                 ~~
+!!! error TS2304: Cannot find name 'we'.
+                                    ~~~
+!!! error TS1005: ';' expected.
+                                    ~~~
+!!! error TS2304: Cannot find name 'had'.
+                                        ~~~~~~~
+!!! error TS1005: ';' expected.
+                                        ~~~~~~~
+!!! error TS2304: Cannot find name 'nothing'.
+                                                ~~~~~~
+!!! error TS1005: ';' expected.
+                                                ~~~~~~
+!!! error TS2304: Cannot find name 'before'.
+                                                       ~~
+!!! error TS1005: ';' expected.
+                                                       ~~
+!!! error TS2304: Cannot find name 'us'.
+    we were all going direct to Heaven, we were all going direct
+    ~~
+!!! error TS2304: Cannot find name 'we'.
+       ~~~~
+!!! error TS1005: ';' expected.
+       ~~~~
+!!! error TS2304: Cannot find name 'were'.
+            ~~~
+!!! error TS1005: ';' expected.
+            ~~~
+!!! error TS2304: Cannot find name 'all'.
+                ~~~~~
+!!! error TS1005: ';' expected.
+                ~~~~~
+!!! error TS2304: Cannot find name 'going'.
+                      ~~~~~~
+!!! error TS1005: ';' expected.
+                      ~~~~~~
+!!! error TS2304: Cannot find name 'direct'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'to'.
+                                ~~~~~~
+!!! error TS1005: ';' expected.
+                                ~~~~~~
+!!! error TS2304: Cannot find name 'Heaven'.
+                                        ~~
+!!! error TS2304: Cannot find name 'we'.
+                                           ~~~~
+!!! error TS1005: ';' expected.
+                                           ~~~~
+!!! error TS2304: Cannot find name 'were'.
+                                                ~~~
+!!! error TS1005: ';' expected.
+                                                ~~~
+!!! error TS2304: Cannot find name 'all'.
+                                                    ~~~~~
+!!! error TS1005: ';' expected.
+                                                    ~~~~~
+!!! error TS2304: Cannot find name 'going'.
+                                                          ~~~~~~
+!!! error TS1005: ';' expected.
+                                                          ~~~~~~
+!!! error TS2304: Cannot find name 'direct'.
+    the other way--in short, the period was so far like the present
+    ~~~
+!!! error TS2304: Cannot find name 'the'.
+        ~~~~~
+!!! error TS1005: ';' expected.
+        ~~~~~
+!!! error TS2304: Cannot find name 'other'.
+              ~~~
+!!! error TS1005: ';' expected.
+              ~~~
+!!! error TS2304: Cannot find name 'way'.
+                      ~~~~~
+!!! error TS2304: Cannot find name 'short'.
+                             ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                 ~~~~~~
+!!! error TS1005: ';' expected.
+                                 ~~~~~~
+!!! error TS2304: Cannot find name 'period'.
+                                        ~~~
+!!! error TS1005: ';' expected.
+                                        ~~~
+!!! error TS2304: Cannot find name 'was'.
+                                            ~~
+!!! error TS1005: ';' expected.
+                                            ~~
+!!! error TS2304: Cannot find name 'so'.
+                                               ~~~
+!!! error TS1005: ';' expected.
+                                               ~~~
+!!! error TS2304: Cannot find name 'far'.
+                                                   ~~~~
+!!! error TS1005: ';' expected.
+                                                   ~~~~
+!!! error TS2304: Cannot find name 'like'.
+                                                        ~~~
+!!! error TS1005: ';' expected.
+                                                        ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                                            ~~~~~~~
+!!! error TS1005: ';' expected.
+                                                            ~~~~~~~
+!!! error TS2304: Cannot find name 'present'.
+    period, that some of its noisiest authorities insisted on its
+    ~~~~~~
+!!! error TS2304: Cannot find name 'period'.
+            ~~~~
+!!! error TS2304: Cannot find name 'that'.
+                 ~~~~
+!!! error TS1005: ';' expected.
+                 ~~~~
+!!! error TS2304: Cannot find name 'some'.
+                      ~~
+!!! error TS1005: ';' expected.
+                      ~~
+!!! error TS2304: Cannot find name 'of'.
+                         ~~~
+!!! error TS1005: ';' expected.
+                         ~~~
+!!! error TS2304: Cannot find name 'its'.
+                             ~~~~~~~~
+!!! error TS1005: ';' expected.
+                             ~~~~~~~~
+!!! error TS2304: Cannot find name 'noisiest'.
+                                      ~~~~~~~~~~~
+!!! error TS1005: ';' expected.
+                                      ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'authorities'.
+                                                  ~~~~~~~~
+!!! error TS1005: ';' expected.
+                                                  ~~~~~~~~
+!!! error TS2304: Cannot find name 'insisted'.
+                                                           ~~
+!!! error TS1005: ';' expected.
+                                                           ~~
+!!! error TS2304: Cannot find name 'on'.
+                                                              ~~~
+!!! error TS1005: ';' expected.
+                                                              ~~~
+!!! error TS2304: Cannot find name 'its'.
+    being received, for good or for evil, in the superlative degree
+    ~~~~~
+!!! error TS2304: Cannot find name 'being'.
+          ~~~~~~~~
+!!! error TS1005: ';' expected.
+          ~~~~~~~~
+!!! error TS2304: Cannot find name 'received'.
+                    ~~~
+!!! error TS1109: Expression expected.
+                        ~~~~
+!!! error TS1005: '(' expected.
+                        ~~~~
+!!! error TS2304: Cannot find name 'good'.
+                             ~~
+!!! error TS1005: ';' expected.
+                             ~~
+!!! error TS2304: Cannot find name 'or'.
+                                ~~~
+!!! error TS1005: ';' expected.
+                                    ~~~~
+!!! error TS1005: '(' expected.
+                                    ~~~~
+!!! error TS2304: Cannot find name 'evil'.
+                                    ~~~~~
+!!! error TS2406: Invalid left-hand side in 'for...in' statement.
+                                          ~~
+!!! error TS1109: Expression expected.
+                                             ~~~
+!!! error TS2304: Cannot find name 'the'.
+                                                 ~~~~~~~~~~~
+!!! error TS1005: ')' expected.
+                                                 ~~~~~~~~~~~
+!!! error TS2304: Cannot find name 'superlative'.
+                                                             ~~~~~~
+!!! error TS1005: ';' expected.
+                                                             ~~~~~~
+!!! error TS2304: Cannot find name 'degree'.
+    of comparison only.
+    ~~
+!!! error TS2304: Cannot find name 'of'.
+       ~~~~~~~~~~
+!!! error TS1005: ';' expected.
+       ~~~~~~~~~~
+!!! error TS2304: Cannot find name 'comparison'.
+                  ~~~~
+!!! error TS1005: ';' expected.
+                  ~~~~
+!!! error TS2304: Cannot find name 'only'.
+                       
+!!! error TS1003: Identifier expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.js b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.js
new file mode 100644
index 0000000000000..30c1de3575183
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/languageServiceExtensions/can-override-all-functions.js
@@ -0,0 +1,117 @@
+//// [atotc.ts]
+It was the best of times, it was the worst of times,
+it was the age of wisdom, it was the age of foolishness,
+it was the epoch of belief, it was the epoch of incredulity,
+it was the season of Light, it was the season of Darkness,
+it was the spring of hope, it was the winter of despair,
+we had everything before us, we had nothing before us,
+we were all going direct to Heaven, we were all going direct
+the other way--in short, the period was so far like the present
+period, that some of its noisiest authorities insisted on its
+being received, for good or for evil, in the superlative degree
+of comparison only.
+
+//// [atotc.js]
+It;
+was;
+the;
+best;
+of;
+times, it;
+was;
+the;
+worst;
+of;
+times,
+    it;
+was;
+the;
+age;
+of;
+wisdom, it;
+was;
+the;
+age;
+of;
+foolishness,
+    it;
+was;
+the;
+epoch;
+of;
+belief, it;
+was;
+the;
+epoch;
+of;
+incredulity,
+    it;
+was;
+the;
+season;
+of;
+Light, it;
+was;
+the;
+season;
+of;
+Darkness,
+    it;
+was;
+the;
+spring;
+of;
+hope, it;
+was;
+the;
+winter;
+of;
+despair,
+    we;
+had;
+everything;
+before;
+us, we;
+had;
+nothing;
+before;
+us,
+    we;
+were;
+all;
+going;
+direct;
+to;
+Heaven, we;
+were;
+all;
+going;
+direct;
+the;
+other;
+way-- in short, the;
+period;
+was;
+so;
+far;
+like;
+the;
+present;
+period, that;
+some;
+of;
+its;
+noisiest;
+authorities;
+insisted;
+on;
+its;
+being;
+received, ;
+for (good; or; )
+    for (evil,  in the)
+        superlative;
+degree;
+of;
+comparison;
+only.;
diff --git a/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.errors.txt b/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.errors.txt
new file mode 100644
index 0000000000000..7d6ee017a2a6b
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.errors.txt
@@ -0,0 +1,8 @@
+error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-semantic-lint'.'.
+error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-syntactic-lint'.'.
+
+
+!!! error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-semantic-lint'.'.
+!!! error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-syntactic-lint'.'.
+==== /hello.ts (0 errors) ====
+    console.log("Hello, world!");/*EOL*/
\ No newline at end of file
diff --git a/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.js b/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.js
new file mode 100644
index 0000000000000..21b4721f1a6ed
--- /dev/null
+++ b/tests/baselines/reference/CompilerHost/reportsFailedLoads/test.js
@@ -0,0 +1,5 @@
+//// [hello.ts]
+console.log("Hello, world!");/*EOL*/
+
+//// [hello.js]
+console.log("Hello, world!"); /*EOL*/
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.errors.txt b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.errors.txt
new file mode 100644
index 0000000000000..86ceab8437d30
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.errors.txt
@@ -0,0 +1,6 @@
+message test-plugin-loaded: Test language service plugin loaded!
+
+
+!!! message test-plugin-loaded: Test language service plugin loaded!
+==== /hello.ts (0 errors) ====
+    console.log("Hello, world!");/*EOL*/
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.js b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.js
new file mode 100644
index 0000000000000..b9f140e37bdde
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-add-program-diagnostics.js
@@ -0,0 +1,5 @@
+//// [hello.ts]
+console.log("Hello, world!");/*EOL*/
+
+//// [hello.js]
+console.log("Hello, world!"); /*EOL*/
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.errors.txt b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.errors.txt
new file mode 100644
index 0000000000000..28ac94a0b59fb
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.errors.txt
@@ -0,0 +1,16 @@
+message program-diagnostics-mutated: Program diagnostics mutated!
+message semantic-diagnostics-mutated: Semantic diagnostics mutated!
+message syntactic-diagnostics-mutated: Syntactic diagnostics mutated!
+/hello-error.ts(1,1): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+/hello-error.ts(1,31): error TS1109: Expression expected.
+
+
+!!! message program-diagnostics-mutated: Program diagnostics mutated!
+!!! message semantic-diagnostics-mutated: Semantic diagnostics mutated!
+!!! message syntactic-diagnostics-mutated: Syntactic diagnostics mutated!
+==== /hello-error.ts (2 errors) ====
+    console.log("Hello, error!") -
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+                                  
+!!! error TS1109: Expression expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.js b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.js
new file mode 100644
index 0000000000000..9650d50b1bb7c
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-chain-services.js
@@ -0,0 +1,5 @@
+//// [hello-error.ts]
+console.log("Hello, error!") -
+
+//// [hello-error.js]
+console.log("Hello, error!") - ;
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.errors.txt b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.errors.txt
new file mode 100644
index 0000000000000..63a20e9cb70e1
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.errors.txt
@@ -0,0 +1,20 @@
+message program-diagnostics-replaced: Program diagnostics replaced!
+message semantic-diagnostics-replaced: Semantic diagnostics replaced!
+message syntactic-diagnostics-replaced: Syntactic diagnostics replaced!
+
+
+!!! message program-diagnostics-replaced: Program diagnostics replaced!
+!!! message semantic-diagnostics-replaced: Semantic diagnostics replaced!
+!!! message syntactic-diagnostics-replaced: Syntactic diagnostics replaced!
+==== /atotc.ts (0 errors) ====
+    It was the best of times, it was the worst of times,
+    it was the age of wisdom, it was the age of foolishness,
+    it was the epoch of belief, it was the epoch of incredulity,
+    it was the season of Light, it was the season of Darkness,
+    it was the spring of hope, it was the winter of despair,
+    we had everything before us, we had nothing before us,
+    we were all going direct to Heaven, we were all going direct
+    the other way--in short, the period was so far like the present
+    period, that some of its noisiest authorities insisted on its
+    being received, for good or for evil, in the superlative degree
+    of comparison only.
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.js b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.js
new file mode 100644
index 0000000000000..0a4b5fe31af18
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-all-functions.js
@@ -0,0 +1,117 @@
+//// [atotc.ts]
+It was the best of times, it was the worst of times,
+it was the age of wisdom, it was the age of foolishness,
+it was the epoch of belief, it was the epoch of incredulity,
+it was the season of Light, it was the season of Darkness,
+it was the spring of hope, it was the winter of despair,
+we had everything before us, we had nothing before us,
+we were all going direct to Heaven, we were all going direct
+the other way--in short, the period was so far like the present
+period, that some of its noisiest authorities insisted on its
+being received, for good or for evil, in the superlative degree
+of comparison only.
+
+//// [atotc.js]
+It;
+was;
+the;
+best;
+of;
+times, it;
+was;
+the;
+worst;
+of;
+times,
+    it;
+was;
+the;
+age;
+of;
+wisdom, it;
+was;
+the;
+age;
+of;
+foolishness,
+    it;
+was;
+the;
+epoch;
+of;
+belief, it;
+was;
+the;
+epoch;
+of;
+incredulity,
+    it;
+was;
+the;
+season;
+of;
+Light, it;
+was;
+the;
+season;
+of;
+Darkness,
+    it;
+was;
+the;
+spring;
+of;
+hope, it;
+was;
+the;
+winter;
+of;
+despair,
+    we;
+had;
+everything;
+before;
+us, we;
+had;
+nothing;
+before;
+us,
+    we;
+were;
+all;
+going;
+direct;
+to;
+Heaven, we;
+were;
+all;
+going;
+direct;
+the;
+other;
+way-- in short, the;
+period;
+was;
+so;
+far;
+like;
+the;
+present;
+period, that;
+some;
+of;
+its;
+noisiest;
+authorities;
+insisted;
+on;
+its;
+being;
+received, ;
+for (good; or; )
+    for (evil,  in the)
+        superlative;
+degree;
+of;
+comparison;
+only.;
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.errors.txt b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.errors.txt
new file mode 100644
index 0000000000000..5402510857df8
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.errors.txt
@@ -0,0 +1,10 @@
+/hello-error.ts(1,1): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+/hello-error.ts(1,31): error TS1109: Expression expected.
+
+
+==== /hello-error.ts (2 errors) ====
+    console.log("Hello, error!") -
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+!!! error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.
+                                  
+!!! error TS1109: Expression expected.
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.js b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.js
new file mode 100644
index 0000000000000..9650d50b1bb7c
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-filter-passthru.js
@@ -0,0 +1,5 @@
+//// [hello-error.ts]
+console.log("Hello, error!") -
+
+//// [hello-error.js]
+console.log("Hello, error!") - ;
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.errors.txt b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.errors.txt
new file mode 100644
index 0000000000000..63a20e9cb70e1
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.errors.txt
@@ -0,0 +1,20 @@
+message program-diagnostics-replaced: Program diagnostics replaced!
+message semantic-diagnostics-replaced: Semantic diagnostics replaced!
+message syntactic-diagnostics-replaced: Syntactic diagnostics replaced!
+
+
+!!! message program-diagnostics-replaced: Program diagnostics replaced!
+!!! message semantic-diagnostics-replaced: Semantic diagnostics replaced!
+!!! message syntactic-diagnostics-replaced: Syntactic diagnostics replaced!
+==== /atotc.ts (0 errors) ====
+    It was the best of times, it was the worst of times,
+    it was the age of wisdom, it was the age of foolishness,
+    it was the epoch of belief, it was the epoch of incredulity,
+    it was the season of Light, it was the season of Darkness,
+    it was the spring of hope, it was the winter of despair,
+    we had everything before us, we had nothing before us,
+    we were all going direct to Heaven, we were all going direct
+    the other way--in short, the period was so far like the present
+    period, that some of its noisiest authorities insisted on its
+    being received, for good or for evil, in the superlative degree
+    of comparison only.
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.js b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.js
new file mode 100644
index 0000000000000..0a4b5fe31af18
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/languageServiceExtensions/can-override-all-functions.js
@@ -0,0 +1,117 @@
+//// [atotc.ts]
+It was the best of times, it was the worst of times,
+it was the age of wisdom, it was the age of foolishness,
+it was the epoch of belief, it was the epoch of incredulity,
+it was the season of Light, it was the season of Darkness,
+it was the spring of hope, it was the winter of despair,
+we had everything before us, we had nothing before us,
+we were all going direct to Heaven, we were all going direct
+the other way--in short, the period was so far like the present
+period, that some of its noisiest authorities insisted on its
+being received, for good or for evil, in the superlative degree
+of comparison only.
+
+//// [atotc.js]
+It;
+was;
+the;
+best;
+of;
+times, it;
+was;
+the;
+worst;
+of;
+times,
+    it;
+was;
+the;
+age;
+of;
+wisdom, it;
+was;
+the;
+age;
+of;
+foolishness,
+    it;
+was;
+the;
+epoch;
+of;
+belief, it;
+was;
+the;
+epoch;
+of;
+incredulity,
+    it;
+was;
+the;
+season;
+of;
+Light, it;
+was;
+the;
+season;
+of;
+Darkness,
+    it;
+was;
+the;
+spring;
+of;
+hope, it;
+was;
+the;
+winter;
+of;
+despair,
+    we;
+had;
+everything;
+before;
+us, we;
+had;
+nothing;
+before;
+us,
+    we;
+were;
+all;
+going;
+direct;
+to;
+Heaven, we;
+were;
+all;
+going;
+direct;
+the;
+other;
+way-- in short, the;
+period;
+was;
+so;
+far;
+like;
+the;
+present;
+period, that;
+some;
+of;
+its;
+noisiest;
+authorities;
+insisted;
+on;
+its;
+being;
+received, ;
+for (good; or; )
+    for (evil,  in the)
+        superlative;
+degree;
+of;
+comparison;
+only.;
diff --git a/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.errors.txt b/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.errors.txt
new file mode 100644
index 0000000000000..7d6ee017a2a6b
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.errors.txt
@@ -0,0 +1,8 @@
+error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-semantic-lint'.'.
+error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-syntactic-lint'.'.
+
+
+!!! error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-semantic-lint'.'.
+!!! error TS6151: Extension loading failed with error 'Error: Host could not locate extension 'test-syntactic-lint'.'.
+==== /hello.ts (0 errors) ====
+    console.log("Hello, world!");/*EOL*/
\ No newline at end of file
diff --git a/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.js b/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.js
new file mode 100644
index 0000000000000..b9f140e37bdde
--- /dev/null
+++ b/tests/baselines/reference/LanguageServiceHost/reportsFailedLoads/test.js
@@ -0,0 +1,5 @@
+//// [hello.ts]
+console.log("Hello, world!");/*EOL*/
+
+//// [hello.js]
+console.log("Hello, world!"); /*EOL*/
diff --git a/tests/baselines/reference/library-reference-12.trace.json b/tests/baselines/reference/library-reference-12.trace.json
index 84144f82729c6..25fb7f618cd70 100644
--- a/tests/baselines/reference/library-reference-12.trace.json
+++ b/tests/baselines/reference/library-reference-12.trace.json
@@ -17,6 +17,7 @@
     "File '/a/node_modules/jquery.ts' does not exist.",
     "File '/a/node_modules/jquery.d.ts' does not exist.",
     "Found 'package.json' at '/a/node_modules/jquery/package.json'.",
+    "Expected type of 'typings' field in 'package.json' to be 'string', got 'undefined'.",
     "'package.json' has 'types' field 'dist/jquery.d.ts' that references '/a/node_modules/jquery/dist/jquery.d.ts'.",
     "File '/a/node_modules/jquery/dist/jquery.d.ts' exist - use it as a name resolution result.",
     "======== Type reference directive 'jquery' was successfully resolved to '/a/node_modules/jquery/dist/jquery.d.ts', primary: false. ========"
diff --git a/tests/baselines/reference/library-reference-2.trace.json b/tests/baselines/reference/library-reference-2.trace.json
index 64cdd8091832f..f8119ea65dbb9 100644
--- a/tests/baselines/reference/library-reference-2.trace.json
+++ b/tests/baselines/reference/library-reference-2.trace.json
@@ -2,12 +2,14 @@
     "======== Resolving type reference directive 'jquery', containing file '/consumer.ts', root directory '/types'. ========",
     "Resolving with primary search path '/types'",
     "Found 'package.json' at '/types/jquery/package.json'.",
+    "Expected type of 'typings' field in 'package.json' to be 'string', got 'undefined'.",
     "'package.json' has 'types' field 'jquery.d.ts' that references '/types/jquery/jquery.d.ts'.",
     "File '/types/jquery/jquery.d.ts' exist - use it as a name resolution result.",
     "======== Type reference directive 'jquery' was successfully resolved to '/types/jquery/jquery.d.ts', primary: true. ========",
     "======== Resolving type reference directive 'jquery', containing file 'test/__inferred type names__.ts', root directory '/types'. ========",
     "Resolving with primary search path '/types'",
     "Found 'package.json' at '/types/jquery/package.json'.",
+    "Expected type of 'typings' field in 'package.json' to be 'string', got 'undefined'.",
     "'package.json' has 'types' field 'jquery.d.ts' that references '/types/jquery/jquery.d.ts'.",
     "File '/types/jquery/jquery.d.ts' exist - use it as a name resolution result.",
     "======== Type reference directive 'jquery' was successfully resolved to '/types/jquery/jquery.d.ts', primary: true. ========"
diff --git a/tests/cases/extensions/available/extension-api/index.ts b/tests/cases/extensions/available/extension-api/index.ts
new file mode 100644
index 0000000000000..853c922fb62eb
--- /dev/null
+++ b/tests/cases/extensions/available/extension-api/index.ts
@@ -0,0 +1,17 @@
+import * as tsi from "typescript";
+
+export abstract class LanguageServiceProvider implements tsi.LanguageServiceProvider {
+    static "extension-kind": tsi.ExtensionKind.LanguageService = "language-service";
+    protected ts: typeof tsi;
+    protected args: any;
+    protected host: tsi.LanguageServiceHost;
+    protected service: tsi.LanguageService;
+    protected registry: tsi.DocumentRegistry;
+    constructor(state: {ts: typeof tsi, args: any, host: tsi.LanguageServiceHost, service: tsi.LanguageService, registry: tsi.DocumentRegistry}) {
+        this.ts = state.ts;
+        this.args = state.args;
+        this.host = state.host;
+        this.service = state.service;
+        this.registry = state.registry;
+    }
+}
diff --git a/tests/cases/extensions/available/extension-api/package.json b/tests/cases/extensions/available/extension-api/package.json
new file mode 100644
index 0000000000000..cbd379499d2ec
--- /dev/null
+++ b/tests/cases/extensions/available/extension-api/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "extension-api",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": "",
+  "types": "index.d.ts"
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-language-service/index.ts b/tests/cases/extensions/available/test-language-service/index.ts
new file mode 100644
index 0000000000000..5b64badb41002
--- /dev/null
+++ b/tests/cases/extensions/available/test-language-service/index.ts
@@ -0,0 +1,16 @@
+import {LanguageServiceProvider} from "extension-api";
+
+export default class extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnosticsFilter(previous) {
+        previous.push({
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Test language service plugin loaded!",
+            category: 2,
+            code: "test-plugin-loaded",
+        });
+        return previous;
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-language-service/package.json b/tests/cases/extensions/available/test-language-service/package.json
new file mode 100644
index 0000000000000..3003fac1bf883
--- /dev/null
+++ b/tests/cases/extensions/available/test-language-service/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "test-language-service",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": ""
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-chain/index.ts b/tests/cases/extensions/available/test-service-chain/index.ts
new file mode 100644
index 0000000000000..e89e4fbfd17b6
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-chain/index.ts
@@ -0,0 +1,71 @@
+import {LanguageServiceProvider} from "extension-api";
+
+export class AddsDiagnostics extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnosticsFilter(previous) {
+        return previous.concat([{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Program diagnostics amended!",
+            category: 2,
+            code: "program-diagnostics-amended",
+        }]);
+    }
+    getSyntacticDiagnosticsFilter(fileName, previous) {
+        return previous.concat([{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Syntactic diagnostics amended!",
+            category: 2,
+            code: "syntactic-diagnostics-amended",
+        }]);
+    }
+    getSemanticDiagnosticsFilter(fileName, previous) {
+        return previous.concat([{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Semantic diagnostics amended!",
+            category: 2,
+            code: "semantic-diagnostics-amended",
+        }]);
+    }
+}
+
+// Since this is exported second, it should be second in the chain. Probably.
+// This is honestly dependent on js host key ordering 
+export class MutatesAddedDiagnostics extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnosticsFilter(previous) {
+        return previous.map(prev => prev.code === "program-diagnostics-amended" ? {
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Program diagnostics mutated!",
+            category: 2,
+            code: "program-diagnostics-mutated",
+        } : prev);
+    }
+    getSyntacticDiagnosticsFilter(fileName, previous) {
+        return previous.map(prev => prev.code === "syntactic-diagnostics-amended" ? {
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Syntactic diagnostics mutated!",
+            category: 2,
+            code: "syntactic-diagnostics-mutated",
+        } : prev);
+    }
+    getSemanticDiagnosticsFilter(fileName, previous) {
+        return previous.map(prev => prev.code === "semantic-diagnostics-amended" ? {
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Semantic diagnostics mutated!",
+            category: 2,
+            code: "semantic-diagnostics-mutated",
+        } : prev);
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-chain/package.json b/tests/cases/extensions/available/test-service-chain/package.json
new file mode 100644
index 0000000000000..e52968c9a299f
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-chain/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "test-service-chain",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": ""
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-filters/index.ts b/tests/cases/extensions/available/test-service-filters/index.ts
new file mode 100644
index 0000000000000..43feaf6bc0983
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-filters/index.ts
@@ -0,0 +1,134 @@
+import {LanguageServiceProvider} from "extension-api";
+
+import * as ts from "typescript";
+
+export default class extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnosticsFilter(previous) {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Program diagnostics replaced!",
+            category: 2,
+            code: "program-diagnostics-replaced",
+        }];
+    }
+    getSyntacticDiagnosticsFilter(fileName, previous) {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Syntactic diagnostics replaced!",
+            category: 2,
+            code: "syntactic-diagnostics-replaced",
+        }];
+    }
+    getSemanticDiagnosticsFilter(fileName, previous) {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Semantic diagnostics replaced!",
+            category: 2,
+            code: "semantic-diagnostics-replaced",
+        }];
+    }
+    getEncodedSyntacticClassificationsFilter(fileName, span, previous) {
+        return {
+            spans: [span.start, span.length, this.ts.endsWith(fileName, "atotc.ts") ? this.ts.ClassificationType.text : this.ts.ClassificationType.comment],
+            endOfLineState: this.ts.EndOfLineState.None
+        };
+    }
+    getEncodedSemanticClassificationsFilter(fileName, span, previous) {
+        return {
+            spans: [span.start, span.length, this.ts.endsWith(fileName, "atotc.ts") ? this.ts.ClassificationType.moduleName : this.ts.ClassificationType.comment],
+            endOfLineState: this.ts.EndOfLineState.None
+        };
+    }
+    getCompletionsAtPositionFilter(fileName, position, previous) {
+        return {
+            isMemberCompletion: false,
+            isNewIdentifierLocation: false,
+            entries: [{name: "fakeCompletion", kind: "", kindModifiers: "", sortText: "fakeCompletion"}]
+        };
+    }
+    getCompletionEntryDetailsFilter(fileName, position, entryName, previous) {
+        return {
+            name: "fakeCompletion",
+            kind: position.toString(),
+            kindModifiers: entryName,
+            displayParts: [],
+            documentation: [],
+        };
+    }
+    getQuickInfoAtPositionFilter(fileName, position, previous) {
+        return {};
+    }
+    getNameOrDottedNameSpanFilter(fileName, startPos, endPos, previous) {
+        return {};
+    }
+    getBreakpointStatementAtPositionFilter(fileName, position, previous) {
+        return {};
+    }
+    getSignatureHelpItemsFilter(fileName, position, previous) {
+        return {
+            items: [],
+            applicableSpan: undefined,
+            selectedItemIndex: undefined,
+            argumentIndex: 0,
+            argumentCount: 0,
+        };
+    }
+    getRenameInfoFilter(fileName, position, previous) {
+        return {};
+    }
+    findRenameLocationsFilter(fileName, position, findInStrings, findInComments, previous) {
+        return {};
+    }
+    getDefinitionAtPositionFilter(fileName, position, previous) {
+        return {};
+    }
+    getTypeDefinitionAtPositionFilter(fileName, position, previous) {
+        return {};
+    }
+    getReferencesAtPositionFilter(fileName, position, previous) {
+        return {};
+    }
+    findReferencesFilter(fileName, position, previous) {
+        return {};
+    }
+    getDocumentHighlightsFilter(fileName, position, filesToSearch, previous) {
+        return {};
+    }
+    getNavigateToItemsFilter(searchValue, maxResultCount, previous) {
+        return {};
+    }
+    getNavigationBarItemsFilter(fileName, previous) {
+        return {};
+    }
+    getOutliningSpansFilter(fileName, previous) {
+        return {};
+    }
+    getTodoCommentsFilter(fileName, descriptors, previous) {
+        return [];
+    }
+    getBraceMatchingAtPositionFilter(fileName, position, previous) {
+        return [];
+    }
+    getIndentationAtPositionFilter(fileName, position, options, previous) {
+        return {};
+    }
+    getFormattingEditsForRangeFilter(fileName, start, end, options, previous) {
+        return {};
+    }
+    getFormattingEditsForDocumentFilter(fileName, options, previous) {
+        return {};
+    }
+    getFormattingEditsAfterKeystrokeFilter(fileName, position, key, options, previous) {
+        return {};
+    }
+    getDocCommentTemplateAtPositionFilter(fileName, position, previous) {
+        return {newText: "/********Yes.*********/", caretOffset: 9};
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-filters/package.json b/tests/cases/extensions/available/test-service-filters/package.json
new file mode 100644
index 0000000000000..4afc14481f05a
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-filters/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "test-service-filters",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": ""
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-overrides/index.ts b/tests/cases/extensions/available/test-service-overrides/index.ts
new file mode 100644
index 0000000000000..f206583f0a029
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-overrides/index.ts
@@ -0,0 +1,133 @@
+import {LanguageServiceProvider} from "extension-api";
+import * as ts from "typescript";
+
+export default class extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnostics() {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Program diagnostics replaced!",
+            category: 2,
+            code: "program-diagnostics-replaced",
+        }];
+    }
+    getSyntacticDiagnostics(fileName) {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Syntactic diagnostics replaced!",
+            category: 2,
+            code: "syntactic-diagnostics-replaced",
+        }];
+    }
+    getSemanticDiagnostics(fileName) {
+        return [{
+            file: undefined,
+            start: undefined,
+            length: undefined,
+            messageText: "Semantic diagnostics replaced!",
+            category: 2,
+            code: "semantic-diagnostics-replaced",
+        }];
+    }
+    getEncodedSyntacticClassifications(fileName, span) {
+        return {
+            spans: [span.start, span.length, this.ts.endsWith(fileName, "atotc.ts") ? this.ts.ClassificationType.text : this.ts.ClassificationType.comment],
+            endOfLineState: this.ts.EndOfLineState.None
+        };
+    }
+    getEncodedSemanticClassifications(fileName, span) {
+        return {
+            spans: [span.start, span.length, this.ts.endsWith(fileName, "atotc.ts") ? this.ts.ClassificationType.moduleName : this.ts.ClassificationType.comment],
+            endOfLineState: this.ts.EndOfLineState.None
+        };
+    }
+    getCompletionsAtPosition(fileName, position) {
+        return {
+            isMemberCompletion: false,
+            isNewIdentifierLocation: false,
+            entries: [{name: "fakeCompletion", kind: "", kindModifiers: "", sortText: "fakeCompletion"}]
+        };
+    }
+    getCompletionEntryDetails(fileName, position, entryName) {
+        return {
+            name: "fakeCompletion",
+            kind: position.toString(),
+            kindModifiers: entryName,
+            displayParts: [],
+            documentation: [],
+        };
+    }
+    getQuickInfoAtPosition(fileName, position) {
+        return {};
+    }
+    getNameOrDottedNameSpan(fileName, startPos, endPos) {
+        return {};
+    }
+    getBreakpointStatementAtPosition(fileName, position) {
+        return {};
+    }
+    getSignatureHelpItems(fileName, position) {
+        return {
+            items: [],
+            applicableSpan: undefined,
+            selectedItemIndex: undefined,
+            argumentIndex: 0,
+            argumentCount: 0,
+        };
+    }
+    getRenameInfo(fileName, position) {
+        return {};
+    }
+    findRenameLocations(fileName, position, findInStrings, findInComments) {
+        return {};
+    }
+    getDefinitionAtPosition(fileName, position) {
+        return {};
+    }
+    getTypeDefinitionAtPosition(fileName, position) {
+        return {};
+    }
+    getReferencesAtPosition(fileName, position) {
+        return {};
+    }
+    findReferences(fileName, position) {
+        return {};
+    }
+    getDocumentHighlights(fileName, position, filesToSearch) {
+        return {};
+    }
+    getNavigateToItems(searchValue, maxResultCount) {
+        return {};
+    }
+    getNavigationBarItems(fileName) {
+        return {};
+    }
+    getOutliningSpans(fileName) {
+        return {};
+    }
+    getTodoComments(fileName, descriptors) {
+        return [];
+    }
+    getBraceMatchingAtPosition(fileName, position) {
+        return [];
+    }
+    getIndentationAtPosition(fileName, position, options) {
+        return {};
+    }
+    getFormattingEditsForRange(fileName, start, end, options) {
+        return {};
+    }
+    getFormattingEditsForDocument(fileName, options) {
+        return {};
+    }
+    getFormattingEditsAfterKeystroke(fileName, position, key, options) {
+        return {};
+    }
+    getDocCommentTemplateAtPosition(fileName, position) {
+        return {newText: "/********Yes.*********/", caretOffset: 9};
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-overrides/package.json b/tests/cases/extensions/available/test-service-overrides/package.json
new file mode 100644
index 0000000000000..92436b41974f2
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-overrides/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "test-service-overrides",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": ""
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-passthru/index.ts b/tests/cases/extensions/available/test-service-passthru/index.ts
new file mode 100644
index 0000000000000..074a90623815d
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-passthru/index.ts
@@ -0,0 +1,14 @@
+import {LanguageServiceProvider} from "extension-api";
+
+export default class extends LanguageServiceProvider {
+    constructor(state) { super(state); }
+    getProgramDiagnosticsFilter(previous) {
+        return previous;
+    }
+    getSyntacticDiagnosticsFilter(fileName, previous) {
+        return previous;
+    }
+    getSemanticDiagnosticsFilter(fileName, previous) {
+        return previous;
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/test-service-passthru/package.json b/tests/cases/extensions/available/test-service-passthru/package.json
new file mode 100644
index 0000000000000..4315a97feec91
--- /dev/null
+++ b/tests/cases/extensions/available/test-service-passthru/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "test-service-passthru",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "author": ""
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/tsconfig.json b/tests/cases/extensions/available/tsconfig.json
new file mode 100644
index 0000000000000..d36035609aafc
--- /dev/null
+++ b/tests/cases/extensions/available/tsconfig.json
@@ -0,0 +1,9 @@
+{
+    "compilerOptions": {
+        // This baseUrl option is useful while writing tests - it lets us 
+        //   pretend all these modules can see one another (as if they were in a node_modules folder)
+        //   since when they're loaded into the virtual fs the test host provides, they _will_ be in a
+        //   node_modules folder
+        "baseUrl": "./"
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/available/typescript/package.json b/tests/cases/extensions/available/typescript/package.json
new file mode 100644
index 0000000000000..14adb10419ff1
--- /dev/null
+++ b/tests/cases/extensions/available/typescript/package.json
@@ -0,0 +1,3 @@
+{
+    "types": "../../../../built/local/typescript.d.ts"
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/fourslash/test.ts b/tests/cases/extensions/fourslash/test.ts
new file mode 100644
index 0000000000000..ee1f6094e6e4e
--- /dev/null
+++ b/tests/cases/extensions/fourslash/test.ts
@@ -0,0 +1,5 @@
+/// <reference path="../../fourslash/fourslash.ts" />
+
+goTo.file("hello.ts");
+goTo.marker("EOL");
+verify.caretAtMarker("EOL");
\ No newline at end of file
diff --git a/tests/cases/extensions/fourslash/verify-overridden-diagnostics.ts b/tests/cases/extensions/fourslash/verify-overridden-diagnostics.ts
new file mode 100644
index 0000000000000..5d3ca5220fd06
--- /dev/null
+++ b/tests/cases/extensions/fourslash/verify-overridden-diagnostics.ts
@@ -0,0 +1,15 @@
+/// <reference path="../../fourslash/fourslash.ts" />
+
+goTo.file("atotc.ts");
+verify.getSemanticDiagnostics(JSON.stringify([{message: "Semantic diagnostics replaced!", category: "message", code: "semantic-diagnostics-replaced"}], undefined, 2));
+verify.getSyntacticDiagnostics(JSON.stringify([{message: "Syntactic diagnostics replaced!", category: "message", code: "syntactic-diagnostics-replaced"}], undefined, 2));
+verify.completionListContains("fakeCompletion", "", undefined, "");
+verify.completionEntryDetailIs("fakeCompletion", "", undefined, "0");
+verify.quickInfoIs(undefined, undefined);
+verify.nameOrDottedNameSpanTextIs("");
+verify.signatureHelpArgumentCountIs(0);
+verify.definitionCountIs(0);
+verify.referencesAre([]);
+verify.todoCommentsInCurrentFile([]);
+verify.noMatchingBracePositionInCurrentFile(0);
+verify.DocCommentTemplate("/********Yes.*********/", 9);
diff --git a/tests/cases/extensions/scenarios/reportsFailedLoads/test.json b/tests/cases/extensions/scenarios/reportsFailedLoads/test.json
new file mode 100644
index 0000000000000..221b4e3a867c0
--- /dev/null
+++ b/tests/cases/extensions/scenarios/reportsFailedLoads/test.json
@@ -0,0 +1,9 @@
+{
+    "inputFiles": [
+        "hello.ts"
+    ],
+    "availableExtensions": [],
+    "compilerOptions": {
+        "extensions": ["test-syntactic-lint", "test-semantic-lint"]
+    }
+}
\ No newline at end of file
diff --git a/tests/cases/extensions/source/atotc.ts b/tests/cases/extensions/source/atotc.ts
new file mode 100644
index 0000000000000..9818c6d2f434c
--- /dev/null
+++ b/tests/cases/extensions/source/atotc.ts
@@ -0,0 +1,11 @@
+It was the best of times, it was the worst of times,
+it was the age of wisdom, it was the age of foolishness,
+it was the epoch of belief, it was the epoch of incredulity,
+it was the season of Light, it was the season of Darkness,
+it was the spring of hope, it was the winter of despair,
+we had everything before us, we had nothing before us,
+we were all going direct to Heaven, we were all going direct
+the other way--in short, the period was so far like the present
+period, that some of its noisiest authorities insisted on its
+being received, for good or for evil, in the superlative degree
+of comparison only.
\ No newline at end of file
diff --git a/tests/cases/extensions/source/hello-error.ts b/tests/cases/extensions/source/hello-error.ts
new file mode 100644
index 0000000000000..251cc73498d66
--- /dev/null
+++ b/tests/cases/extensions/source/hello-error.ts
@@ -0,0 +1 @@
+console.log("Hello, error!") -
\ No newline at end of file
diff --git a/tests/cases/extensions/source/hello.ts b/tests/cases/extensions/source/hello.ts
new file mode 100644
index 0000000000000..97d87624465a4
--- /dev/null
+++ b/tests/cases/extensions/source/hello.ts
@@ -0,0 +1 @@
+console.log("Hello, world!");/*EOL*/
\ No newline at end of file