Wrap Directory.watch on Mac OS for the watcher package.

pkg/watcher/test/ready/mac_os_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.
 
 import 'package:scheduled_test/scheduled_test.dart';
+import 'package:watcher/src/directory_watcher/mac_os.dart';
 import 'package:watcher/watcher.dart';
 
 import 'shared.dart';
 import '../utils.dart';
 
 main() {
   initConfig();
 
-  watcherFactory = (dir) => new PollingDirectoryWatcher(dir);
+  watcherFactory = (dir) => new MacOSDirectoryWatcher(dir);
 
   setUp(createSandbox);
 
   sharedTests();
 }