Add a missing import to mac_os_watcher.

pkg/watcher/test/directory_watcher/mac_os_test.dart

Index: pkg/watcher/test/directory_watcher/mac_os_test.dart
diff --git a/pkg/watcher/test/directory_watcher/mac_os_test.dart b/pkg/watcher/test/directory_watcher/mac_os_test.dart
index 8a30e27e6629af94fae6011ac632fb5a7fe7aacc..7d31221dec73c355143a66cde8c2922bca0f9d9e 100644
--- a/pkg/watcher/test/directory_watcher/mac_os_test.dart
+++ b/pkg/watcher/test/directory_watcher/mac_os_test.dart
@@ -2,6 +2,8 @@
 // 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';