package:observe fix various api issues

pkg/observe/lib/html.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.
 
 // TODO(jmesserly): can we handle this more elegantly?
 // In general, it seems like we want a convenient way to take a Stream plus a
 // getter and convert this into an Observable.
 
 /** Helpers for exposing dart:html as observable data. */
 library observe.html;
 
 import 'dart:html';
 
 import 'observe.dart';
 
 /** An observable version of [window.location.hash]. */
 final ObservableLocationHash windowLocation = new ObservableLocationHash._();
 
-class ObservableLocationHash extends ChangeNotifierBase {
+class ObservableLocationHash extends ChangeNotifier {
+  Object _currentHash;
+
   ObservableLocationHash._() {
     // listen on changes to #hash in the URL
     // Note: listen on both popState and hashChange, because IE9 doesn't support
     // history API. See http://dartbug.com/5483
     // TODO(jmesserly): only listen to these if someone is listening to our
     // changes.
     window.onHashChange.listen(_notifyHashChange);
     window.onPopState.listen(_notifyHashChange);
+
+    _currentHash = hash;
   }
 
   @reflectable String get hash => window.location.hash;
 
   /**
    * Pushes a new URL state, similar to the affect of clicking a link.
    * Has no effect if the [value] already equals [window.location.hash].
    */
   @reflectable void set hash(String value) {
     if (value == hash) return;
 
     window.history.pushState(null, '', value);
     _notifyHashChange(null);
   }
 
   void _notifyHashChange(_) {
-    notifyChange(new PropertyChangeRecord(#hash));
+    var oldValue = _currentHash;
+    _currentHash = hash;
+    notifyPropertyChange(#hash, oldValue, _currentHash);
   }
 }
 
 /** Add or remove CSS class [className] based on the [value]. */
 void updateCssClass(Element element, String className, bool value) {
   if (value == true) {
     element.classes.add(className);
   } else {
     element.classes.remove(className);
   }
 }
 
 /** Bind a CSS class to the observable [object] and property [path]. */
 PathObserver bindCssClass(Element element, String className,
     Observable object, String path) {
 
   return new PathObserver(object, path)..bindSync((value) {
     updateCssClass(element, className, value);
   });
 }