Add a "cache add" command to download packages to the cache.

sdk/lib/_internal/pub/lib/src/command/cache.dart

 // Copyright (c) 2013, 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;
 
 import '../command.dart';
+import 'cache_add.dart';
 import 'cache_list.dart';
 
 /// Handles the `cache` pub command.
 class CacheCommand extends PubCommand {
   String get description => "Work with the system cache.";
   String get usage => "pub cache <subcommand>";
 
   final subcommands = {
+    "add": new CacheAddCommand(),
     "list": new CacheListCommand()
   };
 }