Roll Observatory packages and add a roll script

packages/watcher/test/no_subscription/mac_os_test.dart

+// Copyright (c) 2013, 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.
+
+@TestOn('mac-os')
+
+import 'package:scheduled_test/scheduled_test.dart';
+import 'package:watcher/src/directory_watcher/mac_os.dart';
+
+import 'shared.dart';
+import '../utils.dart';
+
+void main() {
+  watcherFactory = (dir) => new MacOSDirectoryWatcher(dir);
+
+  setUp(createSandbox);
+
+  sharedTests();
+}