Support @Skip as a suite-level annotation.

lib/src/frontend/skip.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.skip;
+
+/// An annotation for marking a test suite as skipped.
+class Skip {
+  /// The reason the test suite is skipped, or `null` if no reason is given.
+  final String reason;
+
+  /// Marks a suite as skipped.
+  ///
+  /// If [reason] is passed, it's included in the test output as the reason the
+  /// test is skipped.
+  const Skip([this.reason]);
+}