[custom_element] fix warning caused by new dart:html methods

pkg/custom_element/lib/custom_element.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.
 
 /**
  * Custom DOM elements.
  *
  * This library provides access to the Polymer project's
  * [Custom Elements]
  * (http://www.polymer-project.org/platform/custom-elements.html)
@@ skipping 80 matching lines @@
    */
   void created() {}
   // Added for analyzer warnings
   @deprecated
   void createdCallback() {}
 
   /** Invoked when this component gets inserted in the DOM tree. */
   void inserted() {}
   @deprecated
   void enteredView() {}
+  @deprecated
+  void attached() {}
 
   /** Invoked when this component is removed from the DOM tree. */
   void removed() {}
   @deprecated
   void leftView() {}
+  @deprecated
+  void detached() {}
 
   /** Invoked when any attribute of the component is modified. */
   void attributeChanged(String name, String oldValue, String newValue) =>
       host.attributeChanged(name, oldValue, newValue);
 
   // TODO(efortuna): Update these when we decide what to do with these
   // properties.
   @deprecated
   String getAttribute(String name) =>
       host.getAttribute(name);
@@ skipping 371 matching lines @@
   Stream<TouchEvent> get onTouchLeave => host.onTouchLeave;
   Stream<TouchEvent> get onTouchMove => host.onTouchMove;
   Stream<TouchEvent> get onTouchStart => host.onTouchStart;
   Stream<TransitionEvent> get onTransitionEnd => host.onTransitionEnd;
 
   // TODO(sigmund): do the normal forwarding when dartbug.com/7919 is fixed.
   Stream<WheelEvent> get onMouseWheel {
     throw new UnsupportedError('onMouseWheel is not supported');
   }
 }