First pass at "pub update" implementation.

utils/pub/command_list.dart

 // Copyright (c) 2012, 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.
 
 /** Handles the `list` pub command. */
 void commandList(PubOptions options, List<String> args) {
   // TODO(rnystrom): Validate args. Right now, this just lists the packages in
   // your cache.
-  final cache = new PackageCache(options.cacheDir);
   cache.listAll().then((packages) {
     for (final package in packages) {
       print(package.name);
     }
   });
 }