add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

pkg/mdv/test/mdv_test_utils.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 observe_utils;
 
 import 'dart:html';
 import 'package:observe/observe.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 28 matching lines @@
   target.dispatchEvent(new Event(type, cancelable: false));
 }
 
 class FooBarModel extends ObservableBase {
   @observable var foo;
   @observable var bar;
 
   FooBarModel([this.foo, this.bar]);
 }
 
+@reflectable
 class FooBarNotifyModel extends ChangeNotifierBase implements FooBarModel {
   var _foo;
   var _bar;
 
   FooBarNotifyModel([this._foo, this._bar]);
 
   get foo => _foo;
   set foo(value) {
     _foo = notifyPropertyChange(#foo, _foo, value);
   }
 
   get bar => _bar;
   set bar(value) {
     _bar = notifyPropertyChange(#bar, _bar, value);
   }
 }
 
 observeTest(name, testCase) => test(name, wrapMicrotask(testCase));
 
 solo_observeTest(name, testCase) => solo_test(name, wrapMicrotask(testCase));