refactor(live-announcer): add unique class to identify live element (#12309)

* Adds a unique class to the live announcer element that is automatically created by the `LiveAnnouncer`. Similarly to the overlay container, the live announcer should be also distinguishable (e.g. for testing)

src/cdk/a11y/live-announcer/live-announcer.spec.ts

@@ -69,7 +69,7 @@ describe('LiveAnnouncer', () => {
       // Call the lifecycle hook manually since Angular won't do it in tests.
       announcer.ngOnDestroy();
 
-      expect(document.body.querySelector('[aria-live]'))
+      expect(document.body.querySelector('.cdk-live-announcer-element'))
           .toBeFalsy('Expected that the aria-live element was remove from the DOM.');
     }));
 
@@ -184,7 +184,7 @@ describe('CdkAriaLive', () => {
 
 
 function getLiveElement(): Element {
-  return document.body.querySelector('[aria-live]')!;
+  return document.body.querySelector('.cdk-live-announcer-element')!;
 }
 
 @Component({template: `<button (click)="announceText('Test')">Announce</button>`})

src/cdk/a11y/live-announcer/live-announcer.ts

@@ -29,15 +29,15 @@ export type AriaLivePoliteness = 'off' | 'polite' | 'assertive';
 
 @Injectable({providedIn: 'root'})
 export class LiveAnnouncer implements OnDestroy {
-  private readonly _liveElement: Element;
+  private readonly _liveElement: HTMLElement;
 
   constructor(
       @Optional() @Inject(LIVE_ANNOUNCER_ELEMENT_TOKEN) elementToken: any,
       @Inject(DOCUMENT) private _document: any) {
 
-    // We inject the live element as `any` because the constructor signature cannot reference
-    // browser globals (HTMLElement) on non-browser environments, since having a class decorator
-    // causes TypeScript to preserve the constructor signature types.
+    // We inject the live element and document as `any` because the constructor signature cannot
+    // reference browser globals (HTMLElement, Document) on non-browser environments, since having
+    // a class decorator causes TypeScript to preserve the constructor signature types.
     this._liveElement = elementToken || this._createLiveElement();
   }
 
@@ -72,10 +72,12 @@ export class LiveAnnouncer implements OnDestroy {
     }
   }
 
-  private _createLiveElement(): Element {
+  private _createLiveElement(): HTMLElement {
     let liveEl = this._document.createElement('div');
 
+    liveEl.classList.add('cdk-live-announcer-element');
     liveEl.classList.add('cdk-visually-hidden');
+
     liveEl.setAttribute('aria-atomic', 'true');
     liveEl.setAttribute('aria-live', 'polite');