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

pkg/watcher/test/directory_watcher/shared.dart

Index: pkg/watcher/test/directory_watcher/shared.dart
diff --git a/pkg/watcher/test/directory_watcher/shared.dart b/pkg/watcher/test/directory_watcher/shared.dart
index fe76a03d384fccd8ed5ac71ce9ba30fee7146f2d..f831196a89533c6cfbfca85064b6c27d4df23b5c 100644
--- a/pkg/watcher/test/directory_watcher/shared.dart
+++ b/pkg/watcher/test/directory_watcher/shared.dart
@@ -51,6 +51,16 @@ sharedTests() {
     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");