Use pub_semver package in pub.

sdk/lib/_internal/pub/test/serve_packages.dart

 // Copyright (c) 2014, 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 serve_packages;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:path/path.dart' as p;
+import 'package:pub_semver/pub_semver.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 import 'package:yaml/yaml.dart';
 
 import '../lib/src/io.dart';
 import '../lib/src/utils.dart';
-import '../lib/src/version.dart';
 import 'descriptor.dart' as d;
 import 'test_pub.dart';
 
 /// The [d.DirectoryDescriptor] describing the server layout of `/api/packages`
 /// on the test server.
 ///
 /// This contains metadata for packages that are being served via
 /// [servePackages]. It's `null` if [servePackages] has not yet been called for
 /// this test.
 d.DirectoryDescriptor _servedApiPackageDir;
@@ skipping 159 matching lines @@
 
 /// A package that's intended to be served.
 class _ServedPackage {
   final Map pubspec;
   final List<d.Descriptor> contents;
 
   Version get version => new Version.parse(pubspec['version']);
 
   _ServedPackage(this.pubspec, this.contents);
 }