package:observe fix various api issues

samples/survey/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 survey.models;
 
 import 'package:polymer/polymer.dart';
 
 final appModel = new SurveyApp();
 
 /*
  * The model for the SurveyApp.
  */
-class SurveyApp extends ObservableBase {
+class SurveyApp extends Observable {
   @observable String title = '';
   @observable String description = '';
   @observable List<Question> questions;
 
   SurveyApp();
 }
 
 /*
  * Model for a survey question. [answerOptions] is the list of options from
  * which the user picks one or more [answers].
  */
-class Question extends ObservableBase {
+class Question extends Observable {
   @observable String text;
   @observable String helpText;
   @observable List<String> answerOptions = [];
   @observable List<String> answers = [];
 
   Question([this.text = '', this.helpText = '']);
 
   bool get isValid => text.isNotEmpty;
 }