pkg/unittest: config refactor

pkg/unittest/lib/html_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.
  */
 library unittest_html_config;
 
 import 'dart:async';
 import 'dart:html';
@@ skipping 145 matching lines @@
         uncaughtError);
   }
 
   void onDone(bool success) {
     _uninstallHandlers();
     window.postMessage('unittest-suite-done', '*');
   }
 }
 
 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);