Make 'useXConfiguration' check for an existing configuration. This is so that it

pkg/unittest/lib/html_config.dart

 // Copyright (c) 2011, 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.
 
 /**
  * A simple unit test library for running tests in a browser.
  */
 library unittest_html_config;
 
 import 'dart:html';
@@ skipping 126 matching lines @@
         uncaughtError);
   }
 
   void onDone(bool success) {
     _uninstallHandlers();
     window.postMessage('unittest-suite-done', '*');
   }
 }
 
 void useHtmlConfiguration([bool isLayoutTest = false]) {
+  if (config != null) return;
   configure(new HtmlConfiguration(isLayoutTest));
 }