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

lib/src/backend/test.dart

Index: lib/src/backend/test.dart
diff --git a/lib/src/backend/test.dart b/lib/src/backend/test.dart
index 85ca2c7da98781bd983d50bec152059322e686aa..0334a8f46a97da06e923e9bb139cd088040da7bc 100644
--- a/lib/src/backend/test.dart
+++ b/lib/src/backend/test.dart
@@ -25,4 +25,8 @@ abstract class Test {
   ///
   /// [suite] is the suite within which this test is being run.
   LiveTest load(Suite suite);
+
+  /// Returns a new copy of this Test with the given [name] and [metadata], if
+  /// passed.
+  Test change({String name, Metadata metadata});
 }