Updating Polymer to derive from custom elements.

samples/third_party/todomvc/web/app.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.app;
 
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'model.dart';
 
-@CustomTag("todo-app")
+@CustomTag('todo-app', type: TodoApp)
 class TodoApp extends PolymerElement {
   @observable AppModel app;
   bool get applyAuthorStyles => true;
 
-  void created() {
-    super.created();
+  factory TodoApp() => new Element.tag('TodoApp');
+
+  TodoApp.created() : super.created() {
     app = appModel;
   }
 
   void addTodo(Event e) {
     e.preventDefault(); // don't submit the form
     InputElement input = getShadowRoot('todo-app').query('#new-todo');
     if (input.value == '') return;
     app.todos.add(new Todo(input.value));
     input.value = '';
   }
 
   void clear() => app.clearDone();
 }