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

pkg/unittest/lib/interactive_html_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.
 
 /**
  * This configuration can be used to rerun selected tests, as well
  * as see diagnostic output from tests. It runs each test in its own
  * IFrame, so the configuration consists of two parts - a 'parent'
  * config that manages all the tests, and a 'child' config for the
  * IFrame that runs the individual tests.
@@ skipping 451 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, depedning on whether the URL has a search part.
  */
 void useInteractiveHtmlConfiguration() {
+  if (config != null) return;
   if (window.location.search == '') { // This is the parent.
     _prepareDom();
     configure(new ParentInteractiveHtmlConfiguration());
   } else {
     configure(new ChildInteractiveHtmlConfiguration());
   }
 }
 
 String _CSS = """
 body {
@@ skipping 187 matching lines @@
 display: block;
 list-style-type: disc;
 -webkit-margin-before: 1em;
 -webkit-margin-after: 1em;
 -webkit-margin-start: 0px;
 -webkit-margin-end: 0px;
 -webkit-padding-start: 40px;
 }
 
   """;