Remove test skip on Windows.

Author: davidmorgan

pkgs/watcher/test/directory_watcher/shared.dart

@@ -280,8 +280,6 @@ void sharedTests() {
         isRemoveEvent('old'),
         isAddEvent('new')
       ]);
-    }, onPlatform: {
-      'windows': const Skip('https://github.com/dart-lang/watcher/issues/21')
     });
 
     test('emits events for many nested files added at once', () async {