Refactor groups to pipe them through to the runner.

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/operating_system.dart';
 import '../../backend/state.dart';
 import '../../backend/suite.dart';
 import '../../backend/test.dart';
+import '../../backend/test_platform.dart';
 import '../../util/multi_channel.dart';
 import '../../util/remote_exception.dart';
 import '../../util/stack_trace_mapper.dart';
 
 /// A test in a running iframe.
-class IframeTest implements Test {
+class IframeTest extends Test {
   final String name;
   final Metadata metadata;
 
   /// The mapper used to map stack traces for errors coming from this test, or
   /// `null`.
   final StackTraceMapper _mapper;
 
   /// The channel used to communicate with the test's [IframeListener].
   final MultiChannel _channel;
 
@@ skipping 35 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 == name && metadata == this.metadata) return this;
-    if (name == null) name = this.name;
-    if (metadata == null) metadata = this.metadata;
-    return new IframeTest(name, metadata, _channel);
+  Test forPlatform(TestPlatform platform, {OperatingSystem os}) {
+    if (!metadata.testOn.evaluate(platform, os: os)) return null;
+    return new IframeTest(
+        name, metadata.forPlatform(platform, os: os), _channel,
+        mapper: _mapper);
   }
 }