Add type annotations to some dart:html closures

tools/dom/templates/html/impl/impl_XMLHttpRequest.darttemplate

 // 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 $LIBRARYNAME;
 
  /**
   * A client-side XHR request for getting data from a URL,
   * formally known as XMLHttpRequest.
   *
@@ skipping 62 matching lines @@
    *         // Do something with the response.
    *     });
    *
    * See also:
    *
    * * [request]
    */
   static Future<String> getString(String url,
       {bool withCredentials, void onProgress(ProgressEvent e)}) {
     return request(url, withCredentials: withCredentials,
-        onProgress: onProgress).then((xhr) => xhr.responseText);
+        onProgress: onProgress).then((HttpRequest xhr) => xhr.responseText);
   }
 
   /**
    * Makes a server POST request with the specified data encoded as form data.
    *
    * This is roughly the POST equivalent of getString. This method is similar
    * to sending a FormData object with broader browser support but limited to
    * String values.
    *
    * If [data] is supplied, the key/value pairs are URI encoded with
@@ skipping 277 matching lines @@
         headers[key] = '${headers[key]}, $value';
       } else {
         headers[key] = value;
       }
     }
     return headers;
   }
 
 $!MEMBERS
 }