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

pkg/unittest/lib/html_layout_config.dart

Index: pkg/unittest/lib/html_layout_config.dart
diff --git a/pkg/unittest/lib/html_layout_config.dart b/pkg/unittest/lib/html_layout_config.dart
index bd809c02dec6e43db0d7595fc6369bb8d32df521..8d8f578ba07e0ce21f14a29646ac0cb31241044c 100644
--- a/pkg/unittest/lib/html_layout_config.dart
+++ b/pkg/unittest/lib/html_layout_config.dart
@@ -305,6 +305,7 @@ void _prepareDom() {
  * 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());