Don't test for file differences in the polling watcher.

pkg/watcher/test/directory_watcher/polling_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/watcher.dart';
 
 import 'shared.dart';
 import '../utils.dart';
 
 main() {
   initConfig();
 
   // Use a short delay to make the tests run quickly.
   watcherFactory = (dir) => new PollingDirectoryWatcher(dir,
       pollingDelay: new Duration(milliseconds: 100));
 
   setUp(createSandbox);
 
   sharedTests();
 
-  test('does not notify if the file contents are unchanged', () {
-    writeFile("a.txt", contents: "same");
-    writeFile("b.txt", contents: "before");
-    startWatcher();
-    writeFile("a.txt", contents: "same");
-    writeFile("b.txt", contents: "after");
-    expectModifyEvent("b.txt");
-  });
-
   test('does not notify if the modification time did not change', () {
     writeFile("a.txt", contents: "before");
     writeFile("b.txt", contents: "before");
     startWatcher();
     writeFile("a.txt", contents: "after", updateModified: false);
     writeFile("b.txt", contents: "after");
     expectModifyEvent("b.txt");
   });
 }