add @MirrorsUsed to observe/polymer/polymer_exprs pkgs

samples/third_party/todomvc/web/editable_label.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.editable_label;
 
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 
 /**
  * Label whose [value] can be edited by double clicking. When editing, it
  * displays a form and input element, otherwise it displays the label.
  */
+// For illustration purposes this type uses Polymer.register instead of
+// CustomTag. We must mark it @reflectable to ensure its members
+// (the event handlers) are preserved and can be referenced from HTML.
+@reflectable
 class EditableLabel extends PolymerElement {
   @observable bool editing = false;
   @published String value = '';
 
   factory EditableLabel() => new Element.tag('editable-label');
 
   EditableLabel.created() : super.created();
 
   bool get applyAuthorStyles => true;
 
@@ skipping 23 matching lines @@
     if (e.keyCode == KeyCode.ESC) {
       editing = false;
     }
   }
 }
 
 @initMethod
 void _init() {
   Polymer.register('editable-label', EditableLabel);
 }