diff --git a/packages/language-service/ivy/test/mock_host.ts b/packages/language-service/ivy/test/mock_host.ts
index f9a39ff3c4..3c322eda21 100644
--- a/packages/language-service/ivy/test/mock_host.ts
+++ b/packages/language-service/ivy/test/mock_host.ts
@@ -36,6 +36,10 @@ export const APP_COMPONENT = join(PROJECT_DIR, 'app', 'app.component.ts');
 export const APP_MAIN = join(PROJECT_DIR, 'app', 'main.ts');
 export const PARSING_CASES = join(PROJECT_DIR, 'app', 'parsing-cases.ts');
 
+const NOOP_FILE_WATCHER: ts.FileWatcher = {
+  close() {}
+};
+
 export const host: ts.server.ServerHost = {
   ...ts.sys,
   readFile(absPath: string, encoding?: string): string |
@@ -44,10 +48,25 @@ export const host: ts.server.ServerHost = {
         // MockTypescriptHost
         return ts.sys.readFile(absPath, encoding);
       },
-  // TODO: Need to cast as never because this is not a proper ServerHost interface.
-  // ts.sys lacks methods like watchFile() and watchDirectory(), but these are not
-  // needed for now.
-} as never;
+  watchFile(path: string, callback: ts.FileWatcherCallback): ts.FileWatcher {
+    return NOOP_FILE_WATCHER;
+  },
+  watchDirectory(path: string, callback: ts.DirectoryWatcherCallback): ts.FileWatcher {
+    return NOOP_FILE_WATCHER;
+  },
+  setTimeout() {
+    throw new Error('setTimeout is not implemented');
+  },
+  clearTimeout() {
+    throw new Error('clearTimeout is not implemented');
+  },
+  setImmediate() {
+    throw new Error('setImmediate is not implemented');
+  },
+  clearImmediate() {
+    throw new Error('clearImmediate is not implemented');
+  },
+};
 
 /**
  * Create a ConfiguredProject and an actual program for the test project located
@@ -73,5 +92,61 @@ export function setup() {
   }
   // The following operation forces a ts.Program to be created.
   const tsLS = project.getLanguageService();
-  return {projectService, project, tsLS};
+  return {
+    service: new MockService(project, projectService),
+    project,
+    tsLS,
+  };
+}
+
+class MockService {
+  private readonly overwritten = new Set<ts.server.NormalizedPath>();
+
+  constructor(
+      private readonly project: ts.server.Project,
+      private readonly ps: ts.server.ProjectService,
+  ) {}
+
+  overwrite(fileName: string, newText: string): string {
+    const scriptInfo = this.getScriptInfo(fileName);
+    this.overwritten.add(scriptInfo.fileName);
+    const snapshot = scriptInfo.getSnapshot();
+    scriptInfo.editContent(0, snapshot.getLength(), preprocess(newText));
+    const sameProgram = this.project.updateGraph();  // clear the dirty flag
+    if (sameProgram) {
+      throw new Error('Project should have updated program after overwrite');
+    }
+    return newText;
+  }
+
+  reset() {
+    if (this.overwritten.size === 0) {
+      return;
+    }
+    for (const fileName of this.overwritten) {
+      const scriptInfo = this.getScriptInfo(fileName);
+      const reloaded = scriptInfo.reloadFromFile();
+      if (!reloaded) {
+        throw new Error(`Failed to reload ${scriptInfo.fileName}`);
+      }
+    }
+    const sameProgram = this.project.updateGraph();
+    if (sameProgram) {
+      throw new Error('Project should have updated program after reset');
+    }
+    this.overwritten.clear();
+  }
+
+  getScriptInfo(fileName: string): ts.server.ScriptInfo {
+    const scriptInfo = this.ps.getScriptInfo(fileName);
+    if (!scriptInfo) {
+      throw new Error(`No existing script info for ${fileName}`);
+    }
+    return scriptInfo;
+  }
+}
+
+const REGEX_CURSOR = /¦/g;
+function preprocess(text: string): string {
+  return text.replace(REGEX_CURSOR, '');
 }
diff --git a/packages/language-service/ivy/test/mock_host_spec.ts b/packages/language-service/ivy/test/mock_host_spec.ts
index 848b0db319..5a2d685f82 100644
--- a/packages/language-service/ivy/test/mock_host_spec.ts
+++ b/packages/language-service/ivy/test/mock_host_spec.ts
@@ -11,9 +11,17 @@ import * as ts from 'typescript/lib/tsserverlibrary';
 import {APP_MAIN, setup, TEST_SRCDIR} from './mock_host';
 
 describe('mock host', () => {
+  const {project, service, tsLS} = setup();
+
+  beforeEach(() => {
+    service.reset();
+  });
+
   it('can load test project from Bazel runfiles', () => {
-    const {project, tsLS} = setup();
     expect(project).toBeInstanceOf(ts.server.ConfiguredProject);
+    const configPath = (project as ts.server.ConfiguredProject).getConfigFilePath();
+    expect(configPath.substring(TEST_SRCDIR.length))
+        .toBe('/angular/packages/language-service/test/project/tsconfig.json');
     const program = tsLS.getProgram();
     expect(program).toBeDefined();
     const sourceFiles = program!.getSourceFiles().map(sf => {
@@ -36,7 +44,6 @@ describe('mock host', () => {
   });
 
   it('produces no TS error for test project', () => {
-    const {project, tsLS} = setup();
     const errors = project.getAllProjectErrors();
     expect(errors).toEqual([]);
     const globalErrors = project.getGlobalProjectErrors();
@@ -44,4 +51,24 @@ describe('mock host', () => {
     const diags = tsLS.getSemanticDiagnostics(APP_MAIN);
     expect(diags).toEqual([]);
   });
+
+  it('can overwrite test file', () => {
+    service.overwrite(APP_MAIN, `const x: string = 0`);
+    const scriptInfo = service.getScriptInfo(APP_MAIN);
+    expect(getText(scriptInfo)).toBe('const x: string = 0');
+  });
+
+  it('can find the cursor', () => {
+    const content = service.overwrite(APP_MAIN, `const fo¦o = 'hello world';`);
+    // content returned by overwrite() is the original content with cursor
+    expect(content).toBe(`const fo¦o = 'hello world';`);
+    const scriptInfo = service.getScriptInfo(APP_MAIN);
+    // script info content should not contain cursor
+    expect(getText(scriptInfo)).toBe(`const foo = 'hello world';`);
+  });
 });
+
+function getText(scriptInfo: ts.server.ScriptInfo): string {
+  const snapshot = scriptInfo.getSnapshot();
+  return snapshot.getText(0, snapshot.getLength());
+}