Add a testOn parameter to test() and group().

lib/src/backend/suite.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 unittest.backend.suite;
 
 import 'dart:collection';
 
 import 'metadata.dart';
+import 'operating_system.dart';
 import 'test.dart';
+import 'test_platform.dart';
 
 /// A test suite.
 ///
 /// A test suite is a set of tests that are intended to be run together and that
 /// share default configuration.
 class Suite {
   /// A description of the platform on which the suite is running, or `null` if
   /// that platform is unknown.
   final String platform;
 
   /// The path to the Dart test suite, or `null` if that path is unknown.
   final String path;
 
   /// The metadata associated with this test suite.
   final Metadata metadata;
 
   /// The tests in the test suite.
   final List<Test> tests;
 
   Suite(Iterable<Test> tests, {this.path, this.platform, Metadata metadata})
       : metadata = metadata == null ? new Metadata() : metadata,
         tests = new UnmodifiableListView<Test>(tests.toList());
 
+  /// Returns a new suite that only contains tests that are valid for the given
+  /// [platform] and [os].
+  ///
+  /// If the suite itself is invalid for [platform] and [os], returns `null`.
+  Suite filter(TestPlatform platform, {OperatingSystem os}) {
+    if (!metadata.testOn.evaluate(platform, os: os)) return null;
+    return change(tests: tests.where((test) {
+      return test.metadata.testOn.evaluate(platform, os: os);
+    }));
+  }
+
   /// Returns a new suite with the given fields updated.
   Suite change({String path, String platform, Metadata metadata,
       Iterable<Test> tests}) {
     if (path == null) path = this.path;
     if (platform == null) platform = this.platform;
     if (metadata == null) metadata = this.metadata;
     if (tests == null) tests = this.tests;
     return new Suite(tests, path: path, platform: platform, metadata: metadata);
   }
 }