introduce ObservableList.listChanges to keep list changes separate from property changes

pkg/observe/lib/src/change_notifier.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 observe.src.change_notifier;
 
 import 'dart:async';
 import 'dart:collection' show UnmodifiableListView;
 import 'package:observe/observe.dart';
 import 'package:observe/src/observable.dart' show notifyPropertyChangeHelper;
@@ skipping 22 matching lines @@
   // for subclasses. Ideally they'd be protected.
   /**
    * Override this method to be called when the [changes] are first observed.
    */
   void observed() {}
 
   /**
    * Override this method to be called when the [changes] are no longer being
    * observed.
    */
-  void unobserved() {}
+  void unobserved() {
+    // Free some memory
+    _changes = null;
+  }
 
   bool deliverChanges() {
     var records = _records;
     _records = null;
     if (hasObservers && records != null) {
       _changes.add(new UnmodifiableListView<ChangeRecord>(records));
       return true;
     }
     return false;
   }
@@ skipping 25 matching lines @@
   void notifyChange(ChangeRecord record) {
     if (!hasObservers) return;
 
     if (_records == null) {
       _records = [];
       scheduleMicrotask(deliverChanges);
     }
     _records.add(record);
   }
 }