Fix bugs where events could be added after watchers were closed.

test/directory_watcher/shared.dart

 // Copyright (c) 2012, 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/utils.dart';
 
 import '../utils.dart';
 
 void sharedTests() {
   test('does not notify for files that already exist when started', () {
     // Make some pre-existing files.
     writeFile("a.txt");
     writeFile("b.txt");
@@ skipping 274 matching lines @@
       deleteFile("dir/sub");
       renameDir("old", "dir/sub");
 
       var events = withPermutations((i, j, k)  =>
           isAddEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
       events.add(isRemoveEvent("dir/sub"));
       inAnyOrder(events);
     });
   });
 }