fix(testing): add discardPeriodicTasks to be used with fakeAsync (#8629)

Closes #8616

modules/@angular/core/test/fake_async_spec.ts

@@ -13,6 +13,7 @@ import {
   flushMicrotasks,
   Log,
   tick,
+  discardPeriodicTasks,
 } from '@angular/core/testing';
 import {TimerWrapper, PromiseWrapper} from '../../router/src/facade/async';
 import {BaseException} from '../../router/src/facade/exceptions';
@@ -204,6 +205,24 @@ export function main() {
            expect(cycles).toEqual(1);
          }));
 
+      it('should clear periodic timers', fakeAsync(() => {
+        var cycles = 0;
+        var id = TimerWrapper.setInterval(() => { cycles++; }, 10);
+
+        tick(10);
+        expect(cycles).toEqual(1);
+
+        discardPeriodicTasks();
+
+        // Tick once to clear out the timer which already started.
+        tick(10);
+        expect(cycles).toEqual(2);
+
+        tick(10);
+        // Nothing should change
+        expect(cycles).toEqual(2);
+      }));
+
       it('should process microtasks before timers', fakeAsync(() => {
            var log = new Log();
 
@@ -259,6 +278,11 @@ export function main() {
         expect(() => { tick(); })
             .toThrowError('The code should be running in the fakeAsync zone to call this function');
       });
+
+      it('calling discardPeriodicTasks should throw', () => {
+        expect(() => { discardPeriodicTasks() ; })
+            .toThrowError('The code should be running in the fakeAsync zone to call this function');
+      });
     });
   });
 }

modules/@angular/core/testing/fake_async.ts

@@ -80,6 +80,15 @@ export function tick(millis: number = 0): void {
   _getFakeAsyncZoneSpec().tick(millis);
 }
 
+/**
+ * Discard all remaining periodic tasks.
+ */
+export function discardPeriodicTasks(): void {
+  let zoneSpec = _getFakeAsyncZoneSpec();
+  let pendingTimers = zoneSpec.pendingPeriodicTimers;
+  zoneSpec.pendingPeriodicTimers.length = 0;
+}
+
 /**
  * Flush any pending microtasks.
  */

modules/@angular/integration_test/public_api_spec.ts

@@ -350,6 +350,7 @@ var CORE_TESTING: string[] = [
   'resetBaseTestProviders',
   'setBaseTestProviders',
   'tick',
+  'discardPeriodicTasks',
   'withProviders',
   'xdescribe',
   'xit'