Use pub_semver package in pub.

sdk/lib/_internal/pub/test/preprocess_test.dart

 // Copyright (c) 2014, 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 pub.test.preprocess_test;
 
+import 'package:pub_semver/pub_semver.dart';
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/preprocess.dart';
-import '../lib/src/version.dart';
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   test("does nothing on a file without preprocessor directives", () {
     var text = '''
 some text
 // normal comment
 // #
@@ skipping 273 matching lines @@
 //# else barback <0.5.0
 //# end
 '''), throwsFormatException);
       });
     });
   });
 }
 
 String _preprocess(String input) =>
     preprocess(input, {'barback': new Version.parse("1.2.3")}, 'source/url');