Updating Polymer to derive from custom elements.

samples/third_party/todomvc/web/app.dart

Index: samples/third_party/todomvc/web/app.dart
diff --git a/samples/third_party/todomvc/web/app.dart b/samples/third_party/todomvc/web/app.dart
index f3382dd6912114675dd5b7d4b504f3a6e3086c9b..4464b1a1e55b7485a08e8c266c2d23eb42f04a5a 100644
--- a/samples/third_party/todomvc/web/app.dart
+++ b/samples/third_party/todomvc/web/app.dart
@@ -8,13 +8,14 @@ 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;
   }