Get rid of all the library tags.

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]);
 }