pkg/observe: wrapMicrotask should return the result the provided testCase

pkg/observe/lib/src/microtask.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * *Warning*: this library is **internal**, and APIs are subject to change.
  *
  * Wraps a callback using [wrapMicrotask] and provides the ability to pump all
  * observable objects and [runAsync] calls via [performMicrotaskCheckpoint].
  */
@@ skipping 31 matching lines @@
     Observable.dirtyCheck();
   }
 }
 
 List<Function> _pending = [];
 
 /**
  * Wraps the [testCase] in a zone that supports [performMicrotaskCheckpoint],
  * and returns the test case.
  */
-wrapMicrotask(void testCase()) {
+wrapMicrotask(testCase()) {
   return () {
-    runZonedExperimental(() {
+    return runZonedExperimental(() {
       try {
-        testCase();
+        return testCase();
       } finally {
         performMicrotaskCheckpoint();
       }
     }, onRunAsync: (callback) => _pending.add(callback));
   };
 }