package:observe fix various api issues

samples/tracker/web/task_element.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 tracker.web.task_element;
 
 import 'package:polymer/polymer.dart';
 import 'package:tracker/models.dart';
 
 @CustomTag('task-element')
-class TaskElement extends PolymerElement with ObservableMixin {
+class TaskElement extends PolymerElement with Observable {
   bool get applyAuthorStyles => true;
   @observable Task task;
   @observable bool usingForm = false;
 
   toggleFormDisplay() {
     usingForm = !usingForm;
   }
 }