[polymer.dart] fix *Changed observers inheriting

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 776 matching lines @@
   /** Call [methodName] method on this object with [args]. */
   invokeMethod(Symbol methodName, List args) =>
       _invokeMethod(this, methodName, args);
 
   /** Call [methodName] method on [receiver] with [args]. */
   static _invokeMethod(receiver, Symbol methodName, List args) {
     // TODO(sigmund): consider making callbacks list all arguments
     // explicitly. Unless VM mirrors are optimized first, this will be expensive
     // once custom elements extend directly from Element (see issue 11108).
     var receiverMirror = reflect(receiver);
-    var method = receiverMirror.type.declarations[methodName];
-    if (method != null && method is MethodMirror) {
+    var method = _findMethod(receiverMirror.type, methodName);
+    if (method != null) {
       // This will either truncate the argument list or extend it with extra
       // null arguments, so it will match the signature.
       // TODO(sigmund): consider accepting optional arguments when we can tell
       // them appart from named arguments (see http://dartbug.com/11334)
       args.length = method.parameters.where((p) => !p.isOptional).length;
     }
     return receiverMirror.invoke(methodName, args).reflectee;
   }
 
+  static MethodMirror _findMethod(ClassMirror type, Symbol name) {
+    do {
+      var member = type.declarations[name];
+      if (member is MethodMirror) return member;
+      type = type.superclass;
+    } while (type != null);
+  }
+
   /**
    * Invokes a function asynchronously.
    * This will call `Platform.flush()` and then return a `new Timer`
    * with the provided [method] and [timeout].
    *
    * If you would prefer to run the callback using
    * [window.requestAnimationFrame], see the [async] method.
    */
   // Dart note: "async" is split into 2 methods so it can have a sensible type
   // signatures. Also removed the various features that don't make sense in a
@@ skipping 255 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);
 }