package:observe fix various api issues

samples/survey/web/elements/selection/selection_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.selection;
 
 import 'dart:html';
 
 import 'package:polymer/polymer.dart';
 
 /**
  * The SelectionElement view. A selection element can be used as a substitute
  * for radios and checkboxes. When passed with a [multi] set to true, this
  * element permits the user to select 0 or more options, otherwise it permits
  * the user to select 0 or 1 option.
  */
 @CustomTag('selection-element')
-class SelectionElement extends PolymerElement with ObservableMixin {
+class SelectionElement extends PolymerElement with Observable {
   bool get applyAuthorStyles => true;
 
   @observable List<String> values = toObservable([]);
   @observable List<int> selectedIndices = toObservable([]);
   @observable bool multi = false;
 
   inserted() {
     super.inserted();
     var root = getShadowRoot('selection-element');
     var items = root.queryAll('li');
@@ skipping 32 matching lines @@
   }
 
   dispatchSelectionResults() {
     List<String> results = [];
     for (var index in selectedIndices) {
       results.add(values[index]);
     }
     dispatchEvent(new CustomEvent('selectionmade', detail: results));
   }
 }