fix(live-announcer): remove announcer element on destroy (#5404)

Implements the `ngOnDestroy` hook on the live announcer service and removes the `_removeLiveElement` method.

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

@@ -23,9 +23,9 @@ describe('LiveAnnouncer', () => {
     })));
 
     afterEach(() => {
-      // In our tests we always remove the current live element, because otherwise we would have
-      // multiple live elements due multiple service instantiations.
-      announcer._removeLiveElement();
+      // In our tests we always remove the current live element, in
+      // order to avoid having multiple announcer elements in the DOM.
+      announcer.ngOnDestroy();
     });
 
     it('should correctly update the announce text', fakeAsync(() => {
@@ -58,13 +58,14 @@ describe('LiveAnnouncer', () => {
       expect(ariaLiveElement.getAttribute('aria-live')).toBe('polite');
     }));
 
-    it('should remove the aria-live element from the DOM', fakeAsync(() => {
+    it('should remove the aria-live element from the DOM on destroy', fakeAsync(() => {
       announcer.announce('Hey Google');
 
       // This flushes our 100ms timeout for the screenreaders.
       tick(100);
 
-      announcer._removeLiveElement();
+      // Call the lifecycle hook manually since Angular won't do it in tests.
+      announcer.ngOnDestroy();
 
       expect(document.body.querySelector('[aria-live]'))
           .toBeFalsy('Expected that the aria-live element was remove from the DOM.');
@@ -85,10 +86,9 @@ describe('LiveAnnouncer', () => {
     });
 
     beforeEach(inject([LiveAnnouncer], (la: LiveAnnouncer) => {
-        announcer = la;
-        ariaLiveElement = getLiveElement();
-      }));
-
+      announcer = la;
+      ariaLiveElement = getLiveElement();
+    }));
 
     it('should allow to use a custom live element', fakeAsync(() => {
       announcer.announce('Custom Element');

src/cdk/a11y/live-announcer.ts

@@ -12,6 +12,7 @@ import {
   Optional,
   Inject,
   SkipSelf,
+  OnDestroy,
 } from '@angular/core';
 import {Platform} from '../platform/platform';
 
@@ -22,8 +23,7 @@ export const LIVE_ANNOUNCER_ELEMENT_TOKEN = new InjectionToken<HTMLElement>('liv
 export type AriaLivePoliteness = 'off' | 'polite' | 'assertive';
 
 @Injectable()
-export class LiveAnnouncer {
-
+export class LiveAnnouncer implements OnDestroy {
   private _liveElement: Element;
 
   constructor(
@@ -57,8 +57,7 @@ export class LiveAnnouncer {
     setTimeout(() => this._liveElement.textContent = message, 100);
   }
 
-  /** Removes the aria-live element from the DOM. */
-  _removeLiveElement() {
+  ngOnDestroy() {
     if (this._liveElement && this._liveElement.parentNode) {
       this._liveElement.parentNode.removeChild(this._liveElement);
     }

src/lib/snack-bar/snack-bar.spec.ts

@@ -47,7 +47,7 @@ describe('MdSnackBar', () => {
 
   afterEach(() => {
     overlayContainerElement.innerHTML = '';
-    liveAnnouncer._removeLiveElement();
+    liveAnnouncer.ngOnDestroy();
   });
 
   beforeEach(() => {
@@ -396,7 +396,7 @@ describe('MdSnackBar with parent MdSnackBar', () => {
 
   afterEach(() => {
     overlayContainerElement.innerHTML = '';
-    liveAnnouncer._removeLiveElement();
+    liveAnnouncer.ngOnDestroy();
   });
 
   it('should close snackBars opened by parent when opening from child MdSnackBar', fakeAsync(() => {