fix(language-service): do not return external template that does not exist (#39898)

There is a bug in tsserver that causes it to crash when it tries to create
script info for an external template that does not exist.

I've submitted an upstream PR
https://github.com/microsoft/TypeScript/pull/41737 to fix this, but before
the commit lands in the stable release, we'll have to workaround the issue
in language service.

Close https://github.com/angular/vscode-ng-language-service/issues/1001

PR Close #39898

packages/language-service/src/ts_plugin.ts

@@ -29,8 +29,16 @@ export function getExternalFiles(project: tss.server.Project): string[] {
     return [];
   }
   const ngLsHost = PROJECT_MAP.get(project);
-  ngLsHost?.getAnalyzedModules();
-  return ngLsHost?.getExternalTemplates() || [];
+  if (ngLsHost === undefined) {
+    return [];
+  }
+  ngLsHost.getAnalyzedModules();
+  return ngLsHost.getExternalTemplates().filter(fileName => {
+    // TODO(kyliau): Remove this when the following PR lands on the version of
+    // TypeScript used in this repo.
+    // https://github.com/microsoft/TypeScript/pull/41737
+    return project.fileExists(fileName);
+  });
 }
 
 export function create(info: tss.server.PluginCreateInfo): tss.LanguageService {

packages/language-service/test/ts_plugin_spec.ts

@@ -21,6 +21,7 @@ const mockProject = {
     },
   },
   hasRoots: () => true,
+  fileExists: () => true,
 } as any;
 
 describe('plugin', () => {
@@ -136,6 +137,12 @@ describe('plugin', () => {
       '/app/test.ng',
     ]);
   });
+
+  it('should not return external template that does not exist', () => {
+    spyOn(mockProject, 'fileExists').and.returnValue(false);
+    const externalTemplates = getExternalFiles(mockProject);
+    expect(externalTemplates.length).toBe(0);
+  });
 });
 
 describe(`with config 'angularOnly = true`, () => {