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

pkg/watcher/test/directory_watcher/mac_os_test.dart

Index: pkg/watcher/test/directory_watcher/mac_os_test.dart
diff --git a/pkg/watcher/test/directory_watcher/mac_os_test.dart b/pkg/watcher/test/directory_watcher/mac_os_test.dart
index 8fc2d3e54dec68e8cc4df849ce9c307a08ae10e9..077787a29e6c624bb364fe556a6384af7ef6d18c 100644
--- a/pkg/watcher/test/directory_watcher/mac_os_test.dart
+++ b/pkg/watcher/test/directory_watcher/mac_os_test.dart
@@ -36,16 +36,6 @@ main() {
     expectAddEvent("dir/newer.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('emits events for many nested files moved out then immediately back in',
       () {
     withPermutations((i, j, k) =>