Use the new test runner.

test/version_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_semver.test.version_test;
 
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import 'utils.dart';
 
 main() {
   test('none', () {
     expect(Version.none.toString(), equals('0.0.0'));
   });
 
@@ skipping 233 matching lines @@
       expect(new Version(1, 2, 3, pre: 'pre', build: 'bui').toString(),
           equals('1.2.3-pre+bui'));
     });
 
     test('preserves leading zeroes', () {
       expect(new Version.parse('001.02.0003-01.dev+pre.002').toString(),
           equals('001.02.0003-01.dev+pre.002'));
     });
   });
 }