Add a missing import to mac_os_watcher.

pkg/watcher/test/directory_watcher/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 'dart:async';
+
 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();
 
@@ skipping 66 matching lines @@
       withPermutations((i, j, k) =>
           expectRemoveEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
     });
 
     inAnyOrder(() {
       withPermutations((i, j, k) =>
           expectAddEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
     });
   });
 }