Use pub_semver package in pub.

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

 // Copyright (c) 2013, 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.
 
+import 'package:pub_semver/pub_semver.dart';
+
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 import '../lib/src/barback.dart' as barback;
-import '../lib/src/version.dart';
 
 main() {
   initConfig();
 
   var constraint = barback.pubConstraints["barback"];
   var current = constraint.min.toString();
   var previous = new Version(constraint.min.major, constraint.min.minor - 1, 0)
       .toString();
   var nextPatch = constraint.min.nextPatch.toString();
   var max = constraint.max.toString();
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     });
 
     d.appDir({"barback": previous}).create();
 
     pubGet(error: """
 Incompatible version constraints on barback:
 - myapp 0.0.0 depends on version $previous
 - pub itself depends on version >=$current <$max""");
   });
 }