Respect top-level @TestOn declarations.

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 expressions indicating which platforms the suite supports.
-  final String testOn;
+  /// The selector indicating which platforms the suite supports.
+  final PlatformSelector testOn;
 
-  Metadata(this.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));
 }