Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/source/hosted.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.source.hosted;
 
 import 'dart:async';
 import 'dart:io' as io;
 import "dart:convert";
 
 import 'package:http/http.dart' as http;
 import 'package:path/path.dart' as path;
+import 'package:pub_semver/pub_semver.dart';
 
 import '../exceptions.dart';
 import '../http.dart';
 import '../io.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../pubspec.dart';
 import '../utils.dart';
-import '../version.dart';
 import 'cached.dart';
 
 /// A package source that gets packages from a package hosting site that uses
 /// the same API as pub.dartlang.org.
 class HostedSource extends CachedSource {
   final name = "hosted";
   final hasMultipleVersions = true;
 
   /// Gets the default URL for the package server for hosted dependencies.
   static String get defaultUrl {
@@ skipping 323 matching lines @@
   var name = description["name"];
   if (name is! String) {
     throw new FormatException("The 'name' key must have a string value.");
   }
 
   var url = description["url"];
   if (url == null) url = HostedSource.defaultUrl;
 
   return new Pair<String, String>(name, url);
 }