Use pub_semver package in pub.

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

+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();
   var sourceSpanVersion = barback.pubConstraints["source_span"].min.toString();
   var stackTraceVersion = barback.pubConstraints["stack_trace"].min.toString();
@@ skipping 114 matching lines @@
     });
     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""");
   });
 }