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

pkg/unittest/lib/html_enhanced_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.
  *
  * Provides enhanced HTML output with collapsible group headers
  * and other at-a-glance information about the test results.
  */
@@ skipping 394 matching lines @@
   }
 
   .unittest-row-description
   {
   }
 
   ''';
 }
 
 void useHtmlEnhancedConfiguration([bool isLayoutTest = false]) {
+  if (config != null) return;
   configure(new HtmlEnhancedConfiguration(isLayoutTest));
 }