Add a function to parse metadata into a usable object.

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;
+
+/// 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;
+
+  Metadata(this.testOn);
+}