pkg/unittest: config refactor

pkg/unittest/lib/html_enhanced_config.dart

-// 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.
 
 /**
  * A simple unit test library for running tests in a browser.
  *
  * Provides enhanced HTML output with collapsible group headers
  * and other at-a-glance information about the test results.
  */
 library unittest_html_enhanced_config;
@@ skipping 399 matching lines @@
   }
 
   .unittest-row-description
   {
   }
 
   ''';
 }
 
 void useHtmlEnhancedConfiguration([bool isLayoutTest = false]) {
-  if (config != null) return;
-  configure(new HtmlEnhancedConfiguration(isLayoutTest));
+  unittestConfiguration = isLayoutTest ? _singletonLayout : _singletonNotLayout;
 }
+
+final _singletonLayout = new HtmlEnhancedConfiguration(true);
+final _singletonNotLayout = new HtmlEnhancedConfiguration(false);