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

pkg/unittest/lib/vm_config.dart

 // Copyright (c) 2012, 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 on the VM.
  */
 library unittest_vm_config;
 
 import 'dart:io';
 import 'unittest.dart';
 
 class VMConfiguration extends Configuration {
   void onDone(bool success) {
     try {
       super.onDone(success);
     } catch (ex) {
       // A non-zero exit code is used by the test infrastructure to detect
       // failure.
       exit(1);
     }
   }
 
   void notifyController(String msg) {}
 }
 
 void useVMConfiguration() {
+  if (config != null) return;
   configure(new VMConfiguration());
 }