Add a function to parse metadata into a usable object.

lib/src/frontend/test_on.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.frontend.test_on;
+
+/// An annotation indicating which platforms a test or test suite supports.
+///
+/// For the full syntax of [expression], see [the README][readme].
+///
+/// [readme]: https://github.com/dart-lang/unittest/#readme
+class TestOn {
+  /// The expression specifying the platform.
+  final String expression;
+
+  const TestOn(this.expression);
+}