Use pub_semver package in pub.

sdk/lib/_internal/pub_generated/lib/src/command/cache_add.dart

 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';
 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() {
     commandParser.addFlag(
         "all",
         help: "Install all matching versions.",
@@ skipping 37 matching lines @@
       if (commandOptions["all"]) {
         versions.sort();
         return Future.forEach(versions, downloadVersion);
       } else {
         versions.sort(Version.prioritize);
         return downloadVersion(versions.last);
       }
     });
   }
 }