Support an @OnPlatform annotation.

lib/src/frontend/on_platform.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 test.frontend.on_platform;
+
+/// An annotation for platform-specific customizations for a test suite.
+///
+/// See [the README][onPlatform].
+///
+/// [onPlatform]: https://github.com/dart-lang/test/blob/master/README.md#platform-specific-configuration
+class OnPlatform {
+  final Map<String, dynamic> annotationsByPlatform;
+
+  const OnPlatform(this.annotationsByPlatform);
+}