Fix warnings and errors detected by dart2js.

lib/web_ui.dart

 // Copyright (c) 2012, 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.
 
 /**
  * This library exports all of the commonly used functions and types for
  * building UI's. It is equivalent to the following imports:
  *
  *     import 'package:web_ui/observe.dart';
  *     import 'package:web_ui/safe_html.dart';
@@ skipping 105 matching lines @@
 
   /** Invoked when this component gets inserted in the DOM tree. */
   void inserted() {}
 
   /** Invoked when this component is removed from the DOM tree. */
   void removed() {}
 
   // TODO(jmesserly): how do we implement this efficiently?
   // See https://github.com/dart-lang/web-ui/issues/37
   /** Invoked when any attribute of the component is modified. */
-  void attributeChanged(String name, String oldValue, String newValue) {}
+  void attributeChanged(String name, String oldValue) {}
 
 
   /**
    * **Note**: This is an implementation helper and should not need to be called
    * from your code.
    *
    * If [ShadowRoot.supported] or [useShadowDom] is false, this distributes
    * children to the insertion points of the emulated ShadowRoot.
    * This is an implementation helper and should not need to be called from your
    * code.
@@ skipping 204 matching lines @@
  * traversal.
  */
 bool useShadowDom = false;
 
 bool get _realShadowRoot => useShadowDom && ShadowRoot.supported;
 
 /**
  * Set this to the [Element] to be used as the root for the Web UI app.
  */
 Element webUIRoot = document.body;