Get rid of all the library tags.

lib/src/backend/closed_exception.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.backend.closed_exception;
-
 /// An exception thrown by various front-end methods when the test framework has
 /// been closed and a test must shut down as soon as possible.
 class ClosedException implements Exception {
   ClosedException();
 
   String toString() => "This test has been closed.";
 }