Add a function to parse metadata into a usable object.

lib/src/frontend/test_on.dart

Index: lib/src/frontend/test_on.dart
diff --git a/lib/src/frontend/test_on.dart b/lib/src/frontend/test_on.dart
new file mode 100644
index 0000000000000000000000000000000000000000..d5ed975400b9a10c89129028b81faba548d62f3a
--- /dev/null
+++ b/lib/src/frontend/test_on.dart
@@ -0,0 +1,17 @@
+// 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);
+}