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

pkg/unittest/lib/interactive_html_config.dart

Index: pkg/unittest/lib/interactive_html_config.dart
diff --git a/pkg/unittest/lib/interactive_html_config.dart b/pkg/unittest/lib/interactive_html_config.dart
index fa3c283bbc09b3c90483b3feda1869a2696351fb..37beabf75b059786397f2e621041123c646f47a7 100644
--- a/pkg/unittest/lib/interactive_html_config.dart
+++ b/pkg/unittest/lib/interactive_html_config.dart
@@ -469,6 +469,7 @@ void _prepareDom() {
  * 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());