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

pkg/watcher/test/directory_watcher/polling_test.dart

Index: pkg/watcher/test/directory_watcher/polling_test.dart
diff --git a/pkg/watcher/test/directory_watcher/polling_test.dart b/pkg/watcher/test/directory_watcher/polling_test.dart
index 02ed5d215240469164c7d4b916ecab63992e7745..da292073cb9fc640a60000b214555eb7f793c323 100644
--- a/pkg/watcher/test/directory_watcher/polling_test.dart
+++ b/pkg/watcher/test/directory_watcher/polling_test.dart
@@ -19,15 +19,6 @@ main() {
 
   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");