Add an "onPlatform" parameter to test() and group().

lib/src/runner/browser/iframe_test.dart

 // Copyright (c) 2015, 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.
 
 library test.runner.browser.iframe_test;
 
 import '../../backend/live_test.dart';
 import '../../backend/live_test_controller.dart';
 import '../../backend/metadata.dart';
 import '../../backend/state.dart';
@@ skipping 42 matching lines @@
       });
     }, () {
       // Ignore all future messages from the test and complete it immediately.
       // We don't need to tell it to run its tear-down because there's nothing a
       // browser test needs to clean up on the file system anyway.
       testChannel.sink.close();
       if (!controller.completer.isCompleted) controller.completer.complete();
     });
     return controller.liveTest;
   }
+
+  Test change({String name, Metadata metadata}) {
+    if (name == null) name = this.name;
+    if (metadata == null) metadata = this.metadata;
+    return new IframeTest(name, metadata, _channel);
+  }
 }