Make 'useXConfiguration' check for an existing configuration. This is so that it

pkg/unittest/lib/html_individual_config.dart

 // Copyright (c) 2011, 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 unit test library for running groups of tests in a browser, instead of the
  * entire test file. This is especially used for large tests files that have
  * many subtests, so we can mark groups as failing at a finer granularity than
  * the entire test file.
  *
@@ skipping 21 matching lines @@
       } catch (e) {
         print('tried to match "$testGroupName"');
         print('NO_SUCH_TEST');
       }
     }
     super.onStart();
   }
 }
 
 void useHtmlIndividualConfiguration([bool isLayoutTest = false]) {
+  if (unittest.config != null) return;
   unittest.configure(new HtmlIndividualConfiguration(isLayoutTest));
 }