pkg/unittest: config refactor

pkg/unittest/lib/html_config.dart

Index: pkg/unittest/lib/html_config.dart
diff --git a/pkg/unittest/lib/html_config.dart b/pkg/unittest/lib/html_config.dart
index 1b9d12a3ff03d8e8a1aa260bff339cc3de925806..303afb63b7a67b93ade97b491a8866e76cc97d84 100644
--- a/pkg/unittest/lib/html_config.dart
+++ b/pkg/unittest/lib/html_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.
 
@@ -164,6 +164,8 @@ class HtmlConfiguration extends Configuration {
 }
 
 void useHtmlConfiguration([bool isLayoutTest = false]) {
-  if (config != null) return;
-  configure(new HtmlConfiguration(isLayoutTest));
+  unittestConfiguration = isLayoutTest ? _singletonLayout : _singletonNotLayout;
 }
+
+final _singletonLayout = new HtmlConfiguration(true);
+final _singletonNotLayout = new HtmlConfiguration(false);