Fix warnings in library-tests.

tests/lib/async/schedule_microtask3_test.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.
 
 library run_async_test;
 
 import 'dart:async';
-import '../../../pkg/unittest/lib/unittest.dart';
+import 'package:expect/expect.dart';
+import 'package:unittest/unittest.dart';
 
 main() {
   test("run async timer after async test", () {
     // Check that Timers don't run before the async callbacks.
     bool timerCallbackExecuted = false;
 
     Timer.run(expectAsync0(() { timerCallbackExecuted = true; }));
 
     scheduleMicrotask(expectAsync0(() {
       Expect.isFalse(timerCallbackExecuted);
@@ skipping 10 matching lines @@
       scheduleMicrotask(expectAsync0(() {
         Expect.isFalse(timerCallbackExecuted);
       }));
     }));
 
     scheduleMicrotask(expectAsync0(() {
       Expect.isFalse(timerCallbackExecuted);
     }));
   });
 }