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

pkg/unittest/lib/html_layout_config.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.
 
 /**
  * A configuration for running layoutr tests with testrunner.
  * This configuration is similar to the interactive_html_config
  * as it runs each test in its own IFrame. However, where the former
  * recreated the IFrame for each test, here the IFrames are preserved.
  * Furthermore we post a message on completion.
@@ skipping 287 matching lines @@
   if (document.query('#child') == null) {
     document.body.nodes.add(new Element.html("<div id='child'></div>"));
   }
 }
 
 /**
  * Allocate a Configuration. We allocate either a parent or
  * child, depending on whether the URL has a search part.
  */
 void useHtmlLayoutConfiguration() {
+  if (config != null) return;
   if (window.location.search == '') { // This is the parent.
     _prepareDom();
     configure(new ParentHtmlConfiguration());
   } else {
     configure(new ChildHtmlConfiguration());
   }
 }