package:observe fix various api issues

samples/third_party/todomvc/web/todo_row.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.todo_row;
 
 import 'dart:html';
 import 'package:polymer/polymer.dart';
 import 'model.dart';
 
 @CustomTag('todo-row')
-class TodoRow extends LIElement with Polymer, ObservableMixin {
+class TodoRow extends LIElement with Polymer, Observable {
   @published Todo todo;
 
   bool get applyAuthorStyles => true;
 
   factory TodoRow() => new Element.tag('todo-row');
 
   TodoRow.created() : super.created() {
     polymerCreated();
   }
 
   void ready() {
     var root = getShadowRoot("todo-row");
     var label = root.query('#label');
     var item = root.query('.todo-item');
 
     bindCssClass(item, 'completed', this, 'todo.done');
     bindCssClass(item, 'editing', label, 'editing');
   }
 
   void removeTodo() => appModel.todos.remove(todo);
 }