Add a "help" command to Pub.

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.
 
 #library('command_list');
 
 #import('package.dart');
 #import('pub.dart');
 
 /**
  * Handles the `list` pub command. This is mostly just used so we can pull
  * some basic data out of pub in the integration tests. Once pub is more
  * full-featured and has other commands that test everything it does, this
  * may go away.
  */
 class ListCommand extends PubCommand {
   String get description() => 'print the contents of repositories';
 
+  String get usage() => 'pub list';
+
   Future onRun() {
     // TODO(nweiz): also list the contents of the packages directory when it's
     // able to determine the source of its packages (that is, when we have a
     // lockfile).
     return cache.listAll().transform((ids) {
       _printIds('system cache', ids);
     });
   }
 
   _printIds(String title, List<PackageId> ids) {
     ids = new List<PackageId>.from(ids);
     ids.sort((a, b) => a.compareTo(b));
 
     print('From $title:');
     for (var id in ids) {
       print('  $id');
     }
   }
 }