add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

samples/third_party/todomvc/web/model.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.
 
 library todomvc.web.model;
 
 import 'package:polymer/polymer.dart';
 
 final appModel = new AppModel._();
 
+@reflectable
 class AppModel extends ObservableBase {
   final ObservableList<Todo> todos = new ObservableList<Todo>();
   @observable int doneCount;
   @observable int remaining;
   @observable List<Todo> visibleTodos;
   @observable bool hasCompleteTodos;
 
   bool _allChecked;
 
   AppModel._() {
@@ skipping 38 matching lines @@
 }
 
 class Todo extends ObservableBase {
   @observable String task;
   @observable bool done = false;
 
   Todo(this.task);
 
   String toString() => "$task ${done ? '(done)' : '(not done)'}";
 }