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

lib/src/backend/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.backend.test;
 
 import 'live_test.dart';
 import 'metadata.dart';
 import 'suite.dart';
 
 /// A single test.
 ///
 /// A test is immutable and stateless, which means that it can't be run
 /// directly. To run one, load a live version using [Test.load] and run it using
 /// [LiveTest.run].
 abstract class Test {
   /// The name of the test.
   String get name;
 
   /// The metadata for the test.
   Metadata get metadata;
 
   /// Loads a live version of this test, which can be used to run it a single
   /// time.
   ///
   /// [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});
 }