Use pub_semver package in pub.

sdk/lib/_internal/pub/lib/src/command/cache_add.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 pub.command.cache_add;
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
+
 import '../command.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../utils.dart';
-import '../version.dart';
 
 /// Handles the `cache add` pub command.
 class CacheAddCommand extends PubCommand {
   String get description => "Install a package.";
   String get usage =>
       "pub cache add <package> [--version <constraint>] [--all]";
   String get docUrl => "http://dartlang.org/tools/pub/cmd/pub-cache.html";
   bool get takesArguments => true;
 
   CacheAddCommand() {
@@ skipping 62 matching lines @@
         versions.sort();
         return Future.forEach(versions, downloadVersion);
       } else {
         // Pick the best matching version.
         versions.sort(Version.prioritize);
         return downloadVersion(versions.last);
       }
     });
   }
 }