Get rid of all the library tags.

lib/src/frontend/test_on.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.test_on;
-
 /// An annotation indicating which platforms a test suite supports.
 ///
 /// For the full syntax of [expression], see [the README][].
 ///
 /// [the README]: https://github.com/dart-lang/test/#platform-selector-syntax
 class TestOn {
   /// The expression specifying the platform.
   final String expression;
 
   const TestOn(this.expression);
 }