Use the new test runner.

test/version_constraint_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_constraint_test;
 
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 import 'package:pub_semver/pub_semver.dart';
 
 import 'utils.dart';
 
 main() {
   test('any', () {
     expect(VersionConstraint.any.isAny, isTrue);
     expect(VersionConstraint.any, allows(
         new Version.parse('0.0.0-blah'),
@@ skipping 161 matching lines @@
           max: v072.nextBreaking)));
     });
 
     test('toString() uses "^"', () {
       var constraint = new VersionConstraint.compatibleWith(v072);
 
       expect(constraint.toString(), equals('^0.7.2'));
     });
   });
 }