pkg/unittest: config refactor

pkg/unittest/lib/compact_vm_config.dart

 // 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 test configuration that generates a compact 1-line progress bar. The bar is
  * updated in-place before and after each test is executed. If all test pass,
  * you should only see a couple lines in the terminal. If a test fails, the
  * failure is shown and the progress bar continues to be updated below it.
  */
@@ skipping 153 matching lines @@
     var res = text.substring(text.length - maxLength + 4);
     var firstSpace = res.indexOf(' ');
     if (firstSpace > 0) {
       res = res.substring(firstSpace);
     }
     return '...$res';
   }
 }
 
 void useCompactVMConfiguration() {
-  if (config != null) return;
-  configure(new CompactVMConfiguration());
+  unittestConfiguration = _singleton;
 }
+
+final _singleton = new CompactVMConfiguration();