Use pub_semver package in pub.

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

 // Copyright (c) 2012, 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_upgrade_test;
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
 import 'package:unittest/unittest.dart';
 
 import '../lib/src/lock_file.dart';
 import '../lib/src/log.dart' as log;
 import '../lib/src/package.dart';
 import '../lib/src/pubspec.dart';
 import '../lib/src/sdk.dart' as sdk;
 import '../lib/src/source/cached.dart';
 import '../lib/src/system_cache.dart';
 import '../lib/src/utils.dart';
-import '../lib/src/version.dart';
 import '../lib/src/solver/version_solver.dart';
 import 'test_pub.dart';
 
 MockSource source1;
 MockSource source2;
 
 main() {
   initConfig();
 
   // Uncomment this to debug failing tests.
@@ skipping 1482 matching lines @@
   }
 
   var source = "mock1";
   if (match[7] != null) {
     source = match[7];
     if (source == "root") source = null;
   }
 
   return new PackageId(name, source, parsedVersion, description);
 }