Added top level definition to Window.

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;
 
+/**
+ * Represents the top level context object for web scripting.
+ *
+ * In a web browser, a [Window] object represents the actual browser window.
+ * In a multi-tabbed browser, each tab has its own [Window] object. A [Window]
+ * is the container that displays a [Document]'s content. All web scripting
+ * happens within the context of a [Window] object.
+ *
+ * **Note:** that this class represents any window, whereas [LocalWindow] is
+ * used to access the properties and content of the current window.
+ *
+ * See also:
+ *
+ * * [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 Window {
   // Fields.
   Location get location;
   History get history;
 
   bool get closed;
   Window get opener;
   Window get parent;
   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);
 }