package:observe fix various api issues

samples/tracker/web/task_form_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_form_element;
 
 import 'package:polymer/polymer.dart';
 import 'package:tracker/models.dart';
 import 'dart:html';
 import 'dart:math';
 
 @CustomTag('task-form-element')
-class TaskFormElement extends PolymerElement with ObservableMixin {
+class TaskFormElement extends PolymerElement with Observable {
   bool get applyAuthorStyles => true;
   @observable Task task;
   @observable String titleErrorMessage = '';
   @observable int maxTitleLength = Task.MAX_TITLE_LENGTH;
   @observable String descriptionErrorMessage = '';
   @observable int maxDescriptionLength = Task.MAX_DESCRIPTION_LENGTH;
   @observable final List<String> taskStatusOptions = toObservable([
       Task.CURRENT, Task.PENDING, Task.COMPLETED]);
   @observable int statusSelectedIndex = 1;
   @observable String previousStatus = '';
@@ skipping 76 matching lines @@
       appModel.tasks.add(task);
     }
   }
 
   deleteTask() {
     if (window.confirm('Are you sure you want to delete this?')) {
       appModel.tasks.remove(task);
     }
   }
 }