Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/validator/dependency.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.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';
 
 /// A validator that validates a package's dependencies.
 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);
@@ skipping 120 matching lines @@
   String _constraintForVersion(Version version) =>
       '">=$version ${_upperBoundForVersion(version)}"';
 
   /// Returns the suggested upper bound for a dependency that was tested against
   /// [version].
   String _upperBoundForVersion(Version version) {
     if (version.major != 0) return '<${version.major + 1}.0.0';
     return '<${version.major}.${version.minor + 1}.0';
   }
 }