Revert "Removed Window.focus and Window.blur as they are no longer recommended."

sdk/lib/html/src/CrossFrameTypes.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.
 
 part of html;
 
 /**
  * An object representing the top-level context object for web scripting.
  *
  * In a web browser, a [Window] object represents the actual browser window.
@@ skipping 68 matching lines @@
    *     myIFrame.elements.add(innerIFrame);
    *
    *     print(myIframe.contentWindow.top == window) // 'true'
    *     print(innerIFrame.contentWindow.top == window) // 'true'
    *
    *     print(window.top == window) // 'true'
    */
   Window get top;
 
   // Methods.
+  void focus();
+  void blur();
   void close();
   void postMessage(var message, String targetOrigin, [List messagePorts = null]);
 }
 
 abstract class Location {
   void set href(String val);
 }
 
 abstract class History {
   void back();
   void forward();
   void go(int distance);
 }