docs(core): add an example of using ContentChildren

modules/@angular/examples/core/di/ts/contentChildren/content_children_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, ContentChildren, Directive, Input, QueryList} from '@angular/core';
+
+@Directive({selector: 'pane'})
+export class Pane {
+  @Input() id: string;
+}
+
+@Component({
+  selector: 'tab',
+  template: `
+    <div>panes: {{serializedPanes}}</div> 
+  `
+})
+export class Tab {
+  @ContentChildren(Pane) panes: QueryList<Pane>;
+
+  get serializedPanes(): string { return this.panes ? this.panes.map(p => p.id).join(', ') : ''; }
+}
+
+@Component({
+  selector: 'example-app',
+  template: `
+    <tab>
+      <pane id="1"></pane>
+      <pane id="2"></pane>
+      <pane id="3" *ngIf="shouldShow"></pane>
+    </tab>
+    
+    <button (click)="show()">Show 3</button>
+  `,
+})
+export class ContentChildrenComp {
+  shouldShow = false;
+
+  show() { this.shouldShow = true; }
+}
+// #enddocregion

modules/@angular/examples/core/di/ts/contentChildren/e2e_test/content_children_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('contentChildren example', () => {
+  afterEach(verifyNoBrowserErrors);
+  let button: ElementFinder;
+  let result: ElementFinder;
+
+  beforeEach(() => {
+    browser.get('/core/di/ts/contentChildren/index.html');
+    button = element(by.css('button'));
+    result = element(by.css('div'));
+  });
+
+  it('should query content children', () => {
+    expect(result.getText()).toEqual('panes: 1, 2');
+
+    button.click();
+
+    expect(result.getText()).toEqual('panes: 1, 2, 3');
+  });
+});

modules/@angular/examples/core/di/ts/contentChildren/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 {ContentChildrenComp, Pane, Tab} from './content_children_example';
+
+@NgModule({
+  imports: [BrowserModule],
+  declarations: [ContentChildrenComp, Pane, Tab],
+  bootstrap: [ContentChildrenComp]
+})
+export class AppModule {
+}