pkg/unittest: config refactor

pkg/unittest/lib/html_enhanced_config.dart

Index: pkg/unittest/lib/html_enhanced_config.dart
diff --git a/pkg/unittest/lib/html_enhanced_config.dart b/pkg/unittest/lib/html_enhanced_config.dart
index f9b9088d2955f40faaaec4506a0161605ec11633..36e3e002e6834664ee24a2ba1f5acb20b4c9541a 100644
--- a/pkg/unittest/lib/html_enhanced_config.dart
+++ b/pkg/unittest/lib/html_enhanced_config.dart
@@ -1,4 +1,4 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2013, 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.
 
@@ -418,6 +418,8 @@ class HtmlEnhancedConfiguration extends Configuration {
 }
 
 void useHtmlEnhancedConfiguration([bool isLayoutTest = false]) {
-  if (config != null) return;
-  configure(new HtmlEnhancedConfiguration(isLayoutTest));
+  config = isLayoutTest ? _singletonLayout : _singletonNotLayout;
 }
+
+final _singletonLayout = new HtmlEnhancedConfiguration(true);
+final _singletonNotLayout = new HtmlEnhancedConfiguration(false);