Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/source/hosted.dart

 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';
 class HostedSource extends CachedSource {
   final name = "hosted";
   final hasMultipleVersions = true;
   static String get defaultUrl {
     var url = io.Platform.environment["PUB_HOSTED_URL"];
     if (url != null) return url;
     return "https://pub.dartlang.org";
   }
   Future<List<Version>> getVersions(String name, description) {
@@ skipping 204 matching lines @@
     throw new FormatException("The description map must contain a 'name' key.");
   }
   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);
 }