package:observe fix various api issues

samples/tracker/lib/models.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.models;
 
 import 'package:polymer/polymer.dart';
 
 final appModel = new Tracker();
 
 /**
  * A model for the tracker app.
  *
  * [tasks] contains all tasks used in this app.
  */
-class Tracker extends ObservableBase {
+class Tracker extends Observable {
   @observable List<Task> tasks;
 }
 
 
 /**
  * A model for creating a single task.
  *
  * A task can be saved or unsaved. Only a saved task has a taskID.
  *
  * This model defines validation rules for a Task. It is the responsibility of
  * the view layer to validate a task before assigning a taskID to the task. A
  * task with a taskID is considered saved.
  */
-class Task extends ObservableBase {
+class Task extends Observable {
   static bool TITLE_REQUIRED = true;
   static const MAX_TITLE_LENGTH = 40;
   static const MAX_DESCRIPTION_LENGTH = 200;
   static const CURRENT = 'current';
   static const PENDING = 'pending';
   static const COMPLETED = 'completed';
 
   @observable int taskID;
   @observable String title = '';
   @observable String description = '';
   @observable String status = PENDING;
   @observable DateTime createdAt;
   @observable DateTime updatedAt;
 
   Task.unsaved();
 
   Task(this.title, this.description, this.status);
 
   bool get isValid {
     var minTitleLength = TITLE_REQUIRED ? 1 : 0;
     return (title.length >= minTitleLength &&
         title.length < MAX_TITLE_LENGTH &&
         description.length < MAX_DESCRIPTION_LENGTH);
   }
 
   bool get saved => taskID != null;
 }