feat(fakeAsync): flush the microtasks before returning

fixes #2269

modules/angular2/src/test_lib/fake_async.dart

@@ -27,7 +27,7 @@ Function fakeAsync(Function fn) {
       a7 = _u, a8 = _u, a9 = _u]) {
     // runZoned() to install a custom exception handler that re-throws
     return runZoned(() {
-      new quiver.FakeAsync().run((quiver.FakeAsync async) {
+      return new quiver.FakeAsync().run((quiver.FakeAsync async) {
         try {
           _fakeAsync = async;
           List args = [
@@ -42,7 +42,9 @@ Function fakeAsync(Function fn) {
             a8,
             a9
           ].takeWhile((a) => a != _u).toList();
-          return Function.apply(fn, args);
+          var res = Function.apply(fn, args);
+          _fakeAsync.flushMicrotasks();
+          return res;
         } finally {
           _fakeAsync = null;
         }

modules/angular2/src/test_lib/fake_async.ts

@@ -46,7 +46,11 @@ export function fakeAsync(fn: Function): Function {
     ListWrapper.clear(_pendingPeriodicTimers);
     ListWrapper.clear(_pendingTimers);
 
-    var res = fakeAsyncZone.run(() => { var res = fn(... args); });
+    let res = fakeAsyncZone.run(() => {
+      let res = fn(... args);
+      flushMicrotasks();
+      return res;
+    });
 
     if (_pendingPeriodicTimers.length > 0) {
       throw new BaseException(

modules/angular2/test/test_lib/fake_async_spec.ts

@@ -46,6 +46,18 @@ export function main() {
       });
     }
 
+    it('should flush microtasks before returning', () => {
+      var thenRan = false;
+
+      fakeAsync(() => { PromiseWrapper.resolve(null).then(_ => { thenRan = true; }); })();
+
+      expect(thenRan).toEqual(true);
+    });
+
+
+    it('should propagate the return value',
+       () => { expect(fakeAsync(() => 'foo')()).toEqual('foo'); });
+
     describe('Promise', () => {
       it('should run asynchronous code', fakeAsync(() => {
            var thenRan = false;