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

pkg/polymer/lib/src/instance.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 polymer;
 
 /**
  * Use this annotation to publish a field as an attribute. For example:
  *
  *     class MyPlaybackElement extends PolymerElement {
@@ skipping 556 matching lines @@
       }
 
       unregisterObserver('${MirrorSystem.getName(name)}__array');
     }
     // if the new value is an array, being observing it
     if (value is ObservableList) {
       if (_observeLog.isLoggable(Level.FINE)) {
         _observeLog.fine('[$localName] observeArrayValue: register observer '
             '$name');
       }
-      var sub = (value as ObservableList).changes.listen((changes) {
+      var sub = value.listChanges.listen((changes) {
         invokeMethod(callbackName, [old]);
       });
       registerObserver('${MirrorSystem.getName(name)}__array', sub);
     }
   }
 
   bool unbindProperty(String name) => unregisterObserver(name);
 
   void unbindAllProperties() {
     if (_propertyObserver != null) {
@@ skipping 485 matching lines @@
 
 class _PropertyValue {
   Object oldValue, newValue;
   _PropertyValue(this.oldValue);
 }
 
 class _PolymerExpressionsWithEventDelegate extends PolymerExpressions {
   prepareBinding(String path, name, node) =>
       Polymer.prepareBinding(path, name, node, super.prepareBinding);
 }