Fix some analyzer warnings and hints in pkg/watcher.

pkg/watcher/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 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
-import 'dart:async';
-
 sharedTests() {
   test('does not notify for files that already exist when started', () {
     // Make some pre-existing files.
     writeFile("a.txt");
     writeFile("b.txt");
 
     startWatcher();
 
     // Change one after the watcher is running.
     writeFile("b.txt", contents: "modified");
@@ skipping 210 matching lines @@
       deleteFile("dir/sub");
       renameDir("old", "dir/sub");
       inAnyOrder(() {
         expectRemoveEvent("dir/sub");
         withPermutations((i, j, k)  =>
             expectAddEvent("dir/sub/sub-$i/sub-$j/file-$k.txt"));
       });
     });
   });
 }