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

pkg/scheduled_test/test/metatest.dart

Index: pkg/scheduled_test/test/metatest.dart
diff --git a/pkg/scheduled_test/test/metatest.dart b/pkg/scheduled_test/test/metatest.dart
index 4834b6e4cf7eab1e3532fc03858310664f280a8e..9e94d07ff91dfc76ac31fc09d3ca4b6916a60858 100644
--- a/pkg/scheduled_test/test/metatest.dart
+++ b/pkg/scheduled_test/test/metatest.dart
@@ -200,10 +200,9 @@ 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) {
@@ -220,7 +219,4 @@ class _MetaConfiguration extends SimpleConfiguration {
       }).toList()
     });
   }
-
-  void onInit() {}
-  void onDone(bool success) {}
 }