Refactor groups to pipe them through to the runner.

lib/src/runner/vm/isolate_test.dart

Index: lib/src/runner/vm/isolate_test.dart
diff --git a/lib/src/runner/vm/isolate_test.dart b/lib/src/runner/vm/isolate_test.dart
index ecdf7eb0c6b026c744f7cf2c8c56338c5593f302..e4bc535be3eb63bfc7f4e5c4d9fcab4a6671ed0a 100644
--- a/lib/src/runner/vm/isolate_test.dart
+++ b/lib/src/runner/vm/isolate_test.dart
@@ -10,14 +10,16 @@ import 'dart:isolate';
 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/remote_exception.dart';
 import '../../utils.dart';
 
 /// A test in another isolate.
-class IsolateTest implements Test {
+class IsolateTest extends Test {
   final String name;
   final Metadata metadata;
 
@@ -26,7 +28,6 @@ class IsolateTest implements Test {
 
   IsolateTest(this.name, this.metadata, this._sendPort);
 
-  /// Loads a single runnable instance of this test.
   LiveTest load(Suite suite) {
     var controller;
 
@@ -85,10 +86,9 @@ class IsolateTest implements Test {
     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 IsolateTest(name, metadata, _sendPort);
+  Test forPlatform(TestPlatform platform, {OperatingSystem os}) {
+    if (!metadata.testOn.evaluate(platform, os: os)) return null;
+    return new IsolateTest(
+        name, metadata.forPlatform(platform, os: os), _sendPort);
   }
 }