feat(core): update zone.js to 0.8.10 and expose the flush method (#16860)

`flush()` can now be used from within fakeAsync tests to simulate moving
time forward until all macrotask events have been cleared from the
event queue.

packages/core/testing/src/fake_async.ts

@@ -115,6 +115,20 @@ export function tick(millis: number = 0): void {
   _getFakeAsyncZoneSpec().tick(millis);
 }
 
+/**
+ * Simulates the asynchronous passage of time for the timers in the fakeAsync zone by
+ * draining the macrotask queue until it is empty. The returned value is the milliseconds
+ * of time that would have been elapsed.
+ *
+ * @param maxTurns
+ * @returns {number} The simulated time elapsed, in millis.
+ *
+ * @experimental
+ */
+export function flush(maxTurns?: number): number {
+  return _getFakeAsyncZoneSpec().flush(maxTurns);
+}
+
 /**
  * Discard all remaining periodic tasks.
  *