package:observe fix various api issues

samples/survey/web/elements/question/question_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 survey.web.question;
 
 import 'dart:html';
 
 import 'package:polymer/polymer.dart';
 import 'package:survey/models.dart';
 
 /*
  *  The QuestionElement view. Use this to set the question text, optionally
  *  provide answer options to the user, and pick the widget that the user sees.
  */
 @CustomTag('question-element')
-class QuestionElement extends PolymerElement with ObservableMixin {
+class QuestionElement extends PolymerElement with Observable {
   static const String TEXTFIELD_OPT = 'Use a text field';
   static const String ONE_FROM_MANY_OPT = 'Select one from many options';
   static const String MANY_FROM_MANY_OPT = 'Select many from many options';
 
   static const OPTIONS_MISSING_MESSAGE = "You didn't add any options";
   static const QUESTION_MISSING_MESSAGE = 'You forgot to add the question text';
   static const DELETE_CONFIRM_MESSAGE = 'Are you sure you want to delete '
                                         'this question?';
 
   // The getters are needed because static consts cannot be used in templates.
@@ skipping 89 matching lines @@
   setSelectionAnswers(Event e, var detail, Element sender) {
     e.preventDefault();
     question.answers = detail;
   }
 
   setTextAnswer(Event e, var detail, var sender) {
     e.preventDefault();
     question.answers = [sender.value.trim()];
   }
 }