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

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 61a74cc30efae9e8491efda489a67a439ef50e35..46b2048f55c207b97eddb1af3b017e9d77789d11 100644
--- a/lib/src/runner/vm/isolate_test.dart
+++ b/lib/src/runner/vm/isolate_test.dart
@@ -16,7 +16,7 @@ import '../../backend/test.dart';
 import '../../util/remote_exception.dart';
 
 /// A test in another isolate.
-class IsolateTest implements Test {
+class IsolateTest extends Test {
   final String name;
   final Metadata metadata;
 
@@ -81,4 +81,10 @@ class IsolateTest implements Test {
     });
     return controller.liveTest;
   }
+
+  Test change({String name, Metadata metadata}) {
+    if (name == null) name = this.name;
+    if (metadata == null) metadata = this.metadata;
+    return new IsolateTest(name, metadata, _sendPort);
+  }
 }