implement dirty checking for @observable objects

pkg/observe/lib/src/observable_map.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.
 
 part of observe;
 
 // TODO(jmesserly): this needs to be faster. We currently require multiple
 // lookups per key to get the old value.
 // TODO(jmesserly): this doesn't implement the precise interfaces like
 // LinkedHashMap, SplayTreeMap or HashMap. However it can use them for the
@@ skipping 25 matching lines @@
     var kind = isInsert ? 'insert' : isRemove ? 'remove' : 'set';
     return '#<MapChangeRecord $kind $key>';
   }
 }
 
 /**
  * Represents an observable map of model values. If any items are added,
  * removed, or replaced, then observers that are listening to [changes]
  * will be notified.
  */
-class ObservableMap<K, V> extends ObservableBase implements Map<K, V> {
+class ObservableMap<K, V> extends ChangeNotifierBase implements Map<K, V> {
   static const _LENGTH = const Symbol('length');
 
   final Map<K, V> _map;
 
   /** Creates an observable map. */
   ObservableMap() : _map = new HashMap<K, V>();
 
   /** Creates a new observable map using a [LinkedHashMap]. */
   ObservableMap.linked() : _map = new LinkedHashMap<K, V>();
 
@@ skipping 85 matching lines @@
       });
       notifyPropertyChange(_LENGTH, len, 0);
     }
     _map.clear();
   }
 
   void forEach(void f(K key, V value)) => _map.forEach(f);
 
   String toString() => Maps.mapToString(this);
 }