pkg/unittest: give Configuration a default (blank) implementation

pkg/scheduled_test/test/metatest.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 library for testing test libraries? We must go deeper.
 ///
 /// Since unit testing code tends to use a lot of global state, it can be tough
 /// to test. This library manages it by running each test case in a child
 /// isolate, then reporting the results back to the parent isolate.
 library metatest;
@@ skipping 182 matching lines @@
 
 /// Ensure that the metatest configuration is loaded.
 void _ensureInitialized() {
   unittestConfiguration = _singleton;
 }
 
 final _singleton = new _MetaConfiguration();
 
 /// Special test configuration for use within the child isolates. This hides all
 /// output and reports data back to the parent isolate.
-class _MetaConfiguration extends SimpleConfiguration {
-  final name = "MetaConfiguration";
+class _MetaConfiguration extends Configuration {
 
-  void logTestCaseMesssage(TestCase testCase, String message) {}
+  _MetaConfiguration() : super.blank();
 
   void onSummary(int passed, int failed, int errors, List<TestCase> results,
       String uncaughtError) {
     _replyTo.send({
       "passed": passed,
       "failed": failed,
       "errors": errors,
       "uncaughtError": uncaughtError,
       "results": results.map((testCase) => {
         "description": testCase.description,
         "message": testCase.message,
         "result": testCase.result,
         "stackTrace": testCase.stackTrace
       }).toList()
     });
   }
-
-  void onInit() {}
-  void onDone(bool success) {}
 }