Fixed a lot of dartanalyzer static analysis issues with the html lib.

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 97 matching lines @@
    *     window.location('http://www.mysite.com', 'foo');
    *     // Does not close this window, as the history has changed.
    *     window.close();
    *     print(window.closed()); // 'false'
    *
    * See also:
    *
    * * [Window close discussion](http://www.w3.org/TR/html5/browsers.html#dom-window-close) from the W3C
    */
   void close();
-  void postMessage(var message, String targetOrigin, [List messagePorts = null]);
+  void postMessage(var message, String targetOrigin, [List messagePorts]);
 }
 
 abstract class Location {
   void set href(String val);
 }
 
 abstract class History {
   void back();
   void forward();
   void go(int distance);
 }