Don't test for file differences in the polling 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';
 
 sharedTests() {
   test('does not notify for files that already exist when started', () {
@@ skipping 33 matching lines @@
 
   test('notifies when a file is modified multiple times', () {
     writeFile("file.txt");
     startWatcher();
     writeFile("file.txt", contents: "modified");
     expectModifyEvent("file.txt");
     writeFile("file.txt", contents: "modified again");
     expectModifyEvent("file.txt");
   });
 
+  test('notifies even 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("a.txt");
+    expectModifyEvent("b.txt");
+  });
+
   test('when the watched directory is deleted, removes all files', () {
     writeFile("dir/a.txt");
     writeFile("dir/b.txt");
 
     startWatcher(dir: "dir");
 
     deleteDir("dir");
     inAnyOrder(() {
       expectRemoveEvent("dir/a.txt");
       expectRemoveEvent("dir/b.txt");
@@ skipping 165 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"));
       });
     });
   });
 }