pkg/unittest: config refactor

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 2475d53405943f6ee7c3c31acb7cce6c285b4d0d..cb94063a91f7efc6e57f047c7132398577b092f0 100644
--- a/pkg/unittest/lib/interactive_html_config.dart
+++ b/pkg/unittest/lib/interactive_html_config.dart
@@ -466,15 +466,17 @@ void _prepareDom() {
  * child, depending 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());
+    config = _singletonParent;
   } else {
-    configure(new ChildInteractiveHtmlConfiguration());
+    config = _singletonChild;
   }
 }
 
+final _singletonParent = new ParentInteractiveHtmlConfiguration();
+final _singletonChild = new ChildInteractiveHtmlConfiguration();
+
 String _CSS = """
 body {
 font-family: Arial, sans-serif;