Fix more warnings for dart:html.

tools/dom/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;
 
 /**
  * Top-level container for a browser tab or window.
  *
  * In a web browser, a [WindowBase] object represents any browser window. This
  * object contains the window's state and its relation to other
  * windows, such as which window opened this window.
  *
  * **Note:** This class represents any window, while [Window] is
  * used to access the properties and content of the current window or tab.
  *
  * ## See also
  *
  * * [Window]
  *
  * ## Other resources
  *
  * * [DOM Window](https://developer.mozilla.org/en-US/docs/DOM/window) from MDN.
  * * [Window](http://www.w3.org/TR/Window/) from the W3C.
  */
-abstract class WindowBase implements EventTarget {
+abstract class WindowBase {
   // Fields.
 
   /**
    * The current location of this window.
    *
    *     Location currentLocation = window.location;
    *     print(currentLocation.href); // 'http://www.example.com:80/'
    */
   LocationBase get location;
 
@@ skipping 109 matching lines @@
 
 abstract class LocationBase {
   void set href(String val);
 }
 
 abstract class HistoryBase {
   void back();
   void forward();
   void go(int distance);
 }