feat(mock zone): isAsyncQueueEmpty

lib/mock/zone.dart

@@ -46,6 +46,11 @@ microLeap() {
   }
 }
 
+/**
+ * Returns whether the async queue is empty.
+ */
+isAsyncQueueEmpty() => _asyncQueue.isEmpty;
+
 /**
  * Simulates a clock tick by running any scheduled timers. Can only be used
  * in [async] tests.Clock tick will call [microLeap] to process the microtask

test/mock/zone_spec.dart

@@ -42,7 +42,9 @@ void main() {
         async(() {
           new Future.value('s').then((_) { thenRan = true; });
           expect(thenRan).toBe(false);
+          expect(isAsyncQueueEmpty()).toBe(false);
           microLeap();
+          expect(isAsyncQueueEmpty()).toBe(true);
           expect(thenRan).toBe(true);
           ran = true;
         })();