package:observe fix various api issues

samples/tracker/web/tracker_app.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.web.tracker_app;
 
 import 'package:polymer/polymer.dart';
 import 'package:tracker/models.dart';
 import 'package:tracker/seed.dart' as seed;
 
 @CustomTag('tracker-app')
-class TrackerApp extends PolymerElement with ObservableMixin {
+class TrackerApp extends PolymerElement with Observable {
   bool get applyAuthorStyles => true;
   @observable final ObservableList<Task> tasks = toObservable([]);
   @observable Tracker app;
   @observable Task newTask = new Task.unsaved();
   @observable String searchParam = '';
   @observable bool usingForm = false;
 
   // Buckets for grouping tasks by status.
   @observable List<Task> current = toObservable([]);
   @observable List<Task> pending = toObservable([]);
@@ skipping 63 matching lines @@
       if (task.status == Task.CURRENT) {
         current.add(task);
       } else if (task.status == Task.PENDING) {
         pending.add(task);
       } else {
         completed.add(task);
       }
     }
   }
 }