Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/validator/dependency.dart

 library pub.validator.dependency;
 import 'dart:async';
+import 'package:pub_semver/pub_semver.dart';
 import '../entrypoint.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../validator.dart';
-import '../version.dart';
 class DependencyValidator extends Validator {
   DependencyValidator(Entrypoint entrypoint) : super(entrypoint);
   Future validate() {
     return Future.forEach(entrypoint.root.pubspec.dependencies, (dependency) {
       if (dependency.source != "hosted") {
         return _warnAboutSource(dependency);
       }
       if (dependency.constraint.isAny) {
         _warnAboutNoConstraint(dependency);
       } else if (dependency.constraint is Version) {
@@ skipping 83 matching lines @@
             'Without an upper bound, you\'re promising to support '
             '${log.bold("all")} future versions of ${dep.name}.');
   }
   String _constraintForVersion(Version version) =>
       '">=$version ${_upperBoundForVersion(version)}"';
   String _upperBoundForVersion(Version version) {
     if (version.major != 0) return '<${version.major + 1}.0.0';
     return '<${version.major}.${version.minor + 1}.0';
   }
 }