Fixing overrideMimeType on IE10.

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 utility for retrieving data from a URL.
  *
  * HttpRequest can be used to obtain data from http, ftp, and file
@@ skipping 156 matching lines @@
    */
   static bool get supportsLoadEndEvent {
 $if DART2JS
     var xhr = new HttpRequest();
     return JS('bool', '("onloadend" in #)', xhr);
 $else
     return true;
 $endif
   }
 
+  /**
+   * Checks to see if the overrideMimeType method is supported on the current
+   * platform.
+   */
+  static bool get supportsOverrideMimeType {
+$if DART2JS
+    var xhr = new HttpRequest();
+    return JS('bool', '("overrideMimeType" in #)', xhr);
+$else
+    return true;
+$endif
+  }
+
 $!MEMBERS
 }