Updating Polymer to derive from custom elements.

samples/third_party/todomvc/web/editable_label.dart

Index: samples/third_party/todomvc/web/editable_label.dart
diff --git a/samples/third_party/todomvc/web/editable_label.dart b/samples/third_party/todomvc/web/editable_label.dart
index a4d1f08d4e15b2f57ad9c59d0027de53e1d064aa..746e62b7359cbd7073b03b5aaedf737bcdbe3f92 100644
--- a/samples/third_party/todomvc/web/editable_label.dart
+++ b/samples/third_party/todomvc/web/editable_label.dart
@@ -15,6 +15,10 @@ 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;
 
   InputElement get _editBox => getShadowRoot("editable-label").query('#edit');
@@ -48,5 +52,5 @@ class EditableLabel extends PolymerElement {
 
 @initMethod
 void _init() {
-  Polymer.register('editable-label', EditableLabel);
+  Polymer.register('editable-label', type: EditableLabel);
 }