diff --git a/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts b/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
index 503b0a3254e..fda3bec2f7c 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadDocumentsAndEditors.ts
@@ -315,7 +315,7 @@ export class MainThreadDocumentsAndEditors {
 		const mainThreadDocuments = new MainThreadDocuments(this, extHostContext, this._modelService, modeService, this._textFileService, fileService, textModelResolverService, untitledEditorService);
 		extHostContext.set(MainContext.MainThreadDocuments, mainThreadDocuments);
 
-		const mainThreadTextEditors = new MainThreadTextEditors(this, extHostContext, codeEditorService, bulkEditService, this._editorService, this._editorGroupService);
+		const mainThreadTextEditors = new MainThreadTextEditors(this, extHostContext, this._modelService, modeService, codeEditorService, bulkEditService, this._editorService, this._editorGroupService);
 		extHostContext.set(MainContext.MainThreadTextEditors, mainThreadTextEditors);
 
 		// It is expected that the ctor of the state computer calls our `_onDelta`.
diff --git a/src/vs/workbench/api/electron-browser/mainThreadEditors.ts b/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
index 8636585f8c2..dfc675d2ec0 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
@@ -27,6 +27,8 @@ import { ExtHostContext, ExtHostEditorsShape, IExtHostContext, ITextDocumentShow
 import { MainThreadDocumentsAndEditors } from './mainThreadDocumentsAndEditors';
 import { MainThreadTextEditor } from './mainThreadEditor';
 import { IOpenerService } from 'vs/platform/opener/common/opener';
+import { IModeService } from 'vs/editor/common/services/modeService';
+import { IModelService } from 'vs/editor/common/services/modelService';
 
 export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 
@@ -43,6 +45,8 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 	constructor(
 		documentsAndEditors: MainThreadDocumentsAndEditors,
 		extHostContext: IExtHostContext,
+		@IModelService private readonly _modelService: IModelService,
+		@IModeService private readonly _modeService: IModeService,
 		@ICodeEditorService private readonly _codeEditorService: ICodeEditorService,
 		@IBulkEditService private readonly _bulkEditService: IBulkEditService,
 		@IEditorService private readonly _editorService: IEditorService,
@@ -170,6 +174,22 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 		return TPromise.as(null);
 	}
 
+	$trySetLanguageById(id: string, languageId: string): TPromise<void> {
+		let mainThreadEditor = this._documentsAndEditors.getEditor(id);
+		if (!mainThreadEditor) {
+			return TPromise.wrapError(disposed(`TextEditor(${id})`));
+		}
+		let textModel = mainThreadEditor.getModel();
+		let modelService = this._modelService;
+		let modeService = this._modeService;
+		if (!modelService || !modeService) {
+			return TPromise.wrapError(new Error('modeService is null for some unit tests'));
+		}
+		let mode = modeService.getOrCreateModeByLanguageName(languageId);
+		modelService.setMode(textModel, mode);
+		return TPromise.as(null);
+	}
+
 	$trySetDecorations(id: string, key: string, ranges: IDecorationOptions[]): TPromise<void> {
 		key = `${this._instanceId}-${key}`;
 		if (!this._documentsAndEditors.getEditor(id)) {
diff --git a/src/vs/workbench/api/node/extHost.protocol.ts b/src/vs/workbench/api/node/extHost.protocol.ts
index 44947586fd5..d0d4b9c9756 100644
--- a/src/vs/workbench/api/node/extHost.protocol.ts
+++ b/src/vs/workbench/api/node/extHost.protocol.ts
@@ -193,6 +193,7 @@ export interface MainThreadTextEditorsShape extends IDisposable {
 	$tryShowEditor(id: string, position: EditorViewColumn): TPromise<void>;
 	$tryHideEditor(id: string): TPromise<void>;
 	$trySetOptions(id: string, options: ITextEditorConfigurationUpdate): TPromise<void>;
+	$trySetLanguageById(id: string, languageId: string): TPromise<void>;
 	$trySetDecorations(id: string, key: string, ranges: editorCommon.IDecorationOptions[]): TPromise<void>;
 	$trySetDecorationsFast(id: string, key: string, ranges: number[]): TPromise<void>;
 	$tryRevealRange(id: string, range: IRange, revealType: TextEditorRevealType): TPromise<void>;
diff --git a/src/vs/workbench/api/node/extHostTextEditor.ts b/src/vs/workbench/api/node/extHostTextEditor.ts
index 0d9d7c2d9a7..62a1971ab7c 100644
--- a/src/vs/workbench/api/node/extHostTextEditor.ts
+++ b/src/vs/workbench/api/node/extHostTextEditor.ts
@@ -435,6 +435,10 @@ export class ExtHostTextEditor implements vscode.TextEditor {
 		this._trySetSelection();
 	}
 
+	setLanguageById(language_id: string): void {
+		this._runOnProxy(() => this._proxy.$trySetLanguageById(this._id, language_id));
+	}
+
 	setDecorations(decorationType: vscode.TextEditorDecorationType, ranges: Range[] | vscode.DecorationOptions[]): void {
 		this._runOnProxy(
 			() => {
diff --git a/src/vs/workbench/test/electron-browser/api/extHostTextEditor.test.ts b/src/vs/workbench/test/electron-browser/api/extHostTextEditor.test.ts
index ff187f60156..42e2dc3dacb 100644
--- a/src/vs/workbench/test/electron-browser/api/extHostTextEditor.test.ts
+++ b/src/vs/workbench/test/electron-browser/api/extHostTextEditor.test.ts
@@ -60,6 +60,7 @@ suite('ExtHostTextEditorOptions', () => {
 			$removeTextEditorDecorationType: undefined,
 			$tryShowEditor: undefined,
 			$tryHideEditor: undefined,
+			$trySetLanguageById: undefined,
 			$trySetDecorations: undefined,
 			$trySetDecorationsFast: undefined,
 			$tryRevealRange: undefined,
diff --git a/src/vs/workbench/test/electron-browser/api/mainThreadEditors.test.ts b/src/vs/workbench/test/electron-browser/api/mainThreadEditors.test.ts
index fdb4b5575c3..5bc1131da84 100644
--- a/src/vs/workbench/test/electron-browser/api/mainThreadEditors.test.ts
+++ b/src/vs/workbench/test/electron-browser/api/mainThreadEditors.test.ts
@@ -94,13 +94,15 @@ suite('MainThreadEditors', () => {
 			}
 		});
 
+		const emptyModeService = null;
+
 		const documentAndEditor = new MainThreadDocumentsAndEditors(
 			rpcProtocol,
 			modelService,
 			textFileService,
 			workbenchEditorService,
 			codeEditorService,
-			null,
+			emptyModeService,
 			fileService,
 			null,
 			null,
@@ -111,6 +113,8 @@ suite('MainThreadEditors', () => {
 		editors = new MainThreadTextEditors(
 			documentAndEditor,
 			SingleProxyRPCProtocol(null),
+			modelService,
+			emptyModeService,
 			codeEditorService,
 			bulkEditService,
 			workbenchEditorService,