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.

tools/public_api_guard/core/testing.d.ts

@@ -32,6 +32,9 @@ export declare function discardPeriodicTasks(): void;
 /** @experimental */
 export declare function fakeAsync(fn: Function): (...args: any[]) => any;
 
+/** @experimental */
+export declare function flush(maxTurns?: number): number;
+
 /** @experimental */
 export declare function flushMicrotasks(): void;