fix(@schematics/angular): fix app shell schematic failure

Fixes #10093

Author: devoto13

packages/schematics/angular/app-shell/index.ts

@@ -29,7 +29,7 @@ import {
   insertImport,
   isImported,
 } from '../utility/ast-utils';
-import { InsertChange } from '../utility/change';
+import { Change, InsertChange } from '../utility/change';
 import { getWorkspace, getWorkspacePath } from '../utility/config';
 import { getAppModulePath } from '../utility/ng-ast-utils';
 import { Schema as AppShellOptions } from './schema';
@@ -221,8 +221,10 @@ function addRouterModule(options: AppShellOptions): Rule {
     const moduleSource = getSourceFile(host, modulePath);
     const changes = addImportToModule(moduleSource, modulePath, 'RouterModule', '@angular/router');
     const recorder = host.beginUpdate(modulePath);
-    changes.forEach((change: InsertChange) => {
-      recorder.insertLeft(change.pos, change.toAdd);
+    changes.forEach((change: Change) => {
+      if (change instanceof InsertChange) {
+        recorder.insertLeft(change.pos, change.toAdd);
+      }
     });
     host.commitUpdate(recorder);
 

packages/schematics/angular/app-shell/index_spec.ts

@@ -78,6 +78,17 @@ describe('App Shell Schematic', () => {
     expect(content).toMatch(/import { RouterModule } from \'@angular\/router\';/);
   });
 
+  it('should not fail when AppModule have imported RouterModule already', () => {
+    const updateRecorder = appTree.beginUpdate('/projects/bar/src/app/app.module.ts');
+    updateRecorder.insertLeft(0, 'import { RouterModule } from \'@angular/router\';');
+    appTree.commitUpdate(updateRecorder);
+
+    const tree = schematicRunner.runSchematic('appShell', defaultOptions, appTree);
+    const filePath = '/projects/bar/src/app/app.module.ts';
+    const content = tree.readContent(filePath);
+    expect(content).toMatch(/import { RouterModule } from \'@angular\/router\';/);
+  });
+
   describe('Add router-outlet', () => {
     function makeInlineTemplate(tree: UnitTestTree, template?: string): void {
       template = template || `