docs(core): add an example of using ContentChild

modules/@angular/examples/core/di/ts/contentChild/content_child_example.ts

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+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+// #docregion Component
+import {Component, ContentChild, Directive, Input} from '@angular/core';
+
+@Directive({selector: 'pane'})
+export class Pane {
+  @Input() id: string;
+}
+
+@Component({
+  selector: 'tab',
+  template: `
+    <div>pane: {{pane?.id}}</div> 
+  `
+})
+export class Tab {
+  @ContentChild(Pane) pane: Pane;
+}
+
+@Component({
+  selector: 'example-app',
+  template: `
+    <tab>
+      <pane id="1" *ngIf="shouldShow"></pane>
+      <pane id="2" *ngIf="!shouldShow"></pane>
+    </tab>
+    
+    <button (click)="toggle()">Toggle</button>
+  `,
+})
+export class ContentChildComp {
+  shouldShow = true;
+
+  toggle() { this.shouldShow = !this.shouldShow; }
+}
+// #enddocregion

modules/@angular/examples/core/di/ts/contentChild/e2e_test/content_child_spec.ts

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+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {verifyNoBrowserErrors} from '../../../../../_common/e2e_util';
+
+describe('contentChild example', () => {
+  afterEach(verifyNoBrowserErrors);
+  let button: ElementFinder;
+  let result: ElementFinder;
+
+  beforeEach(() => {
+    browser.get('/core/di/ts/contentChild/index.html');
+    button = element(by.css('button'));
+    result = element(by.css('div'));
+  });
+
+  it('should query content child', () => {
+    expect(result.getText()).toEqual('pane: 1');
+
+    button.click();
+
+    expect(result.getText()).toEqual('pane: 2');
+  });
+});

modules/@angular/examples/core/di/ts/contentChild/module.ts

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+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {NgModule} from '@angular/core';
+import {BrowserModule} from '@angular/platform-browser';
+import {ContentChildComp, Pane, Tab} from './content_child_example';
+
+@NgModule({
+  imports: [BrowserModule],
+  declarations: [ContentChildComp, Pane, Tab],
+  bootstrap: [ContentChildComp]
+})
+export class AppModule {
+}