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

lib/src/backend/metadata.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.metadata;
 
 import 'platform_selector.dart';
 
 /// Metadata for a test or test suite.
 ///
 /// This metadata comes from declarations on the test itself; it doesn't include
 /// configuration from the user.
 class Metadata {
   /// The selector indicating which platforms the suite supports.
   final PlatformSelector testOn;
 
   /// Creates new Metadata.
   ///
   /// [testOn] defaults to [PlatformSelector.all].
   Metadata({PlatformSelector testOn})
       : testOn = testOn == null ? PlatformSelector.all : testOn;
 
   /// Parses metadata fields from strings.
   ///
   /// Throws a [FormatException] if any field is invalid.
   Metadata.parse({String testOn})
       : this(
           testOn: testOn == null ? null : new PlatformSelector.parse(testOn));
+
+  /// Dezerializes the result of [Metadata.serialize] into a new [Metadata].
+  Metadata.deserialize(serialized)
+      : this.parse(testOn: serialized['testOn']);
+
+  /// Return a new [Metadata] that merges [this] with [other].
+  ///
+  /// If the two [Metadata]s have conflicting properties, [other] wins.
+  Metadata merge(Metadata other) =>
+      new Metadata(testOn: testOn.intersect(other.testOn));
+
+  /// Serializes [this] into a JSON-safe object that can be deserialized using
+  /// [new Metadata.deserialize].
+  serialize() => {
+    'testOn': testOn == PlatformSelector.all ? null : testOn.toString()
+  };
 }