package:observe fix various api issues

samples/word-finder/web/word_finder_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 word_finder.web.word_finder_element;
 
 import 'dart:html';
 import 'dart:async';
 import 'package:polymer/polymer.dart';
 
 @CustomTag('word-finder-element')
-class WordFinderElement extends PolymerElement with ObservableMixin {
+class WordFinderElement extends PolymerElement with Observable {
   bool get applyAuthorStyles => true;
 
   @observable List<String> charsList = toObservable(
       ['A', 'E', 'L', 'S', 'T', '']);
 
   List<String> possibleWords = toObservable([
       'LEAST', 'SETAL', 'SLATE', 'STALE', 'STEAL', 'STELA', 'TAELS', 'TALES',
       'TEALS', 'TESLA', 'AE', 'AL', 'AS', 'AT', 'EL', 'ES', 'ET', 'LA', 'TA',
       'ALE', 'ALS', 'ALT', 'ATE', 'EAT', 'ELS', 'ETA', 'LAS', 'LAT', 'LEA',
       'LES', 'LET', 'SAE', 'SAL', 'SAT', 'SEA', 'SEL', 'SET', 'TAE', 'TAS',
@@ skipping 81 matching lines @@
       }
     }
   }
 
   void _swapChars() {
     var temp = charsList[sourceElementIndex];
     charsList[sourceElementIndex] = charsList[targetElementIndex];
     charsList[targetElementIndex] = temp;
   }
 }