Fix the built-in help command in args.

pkg/args/lib/command_runner.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 args.command_runner;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
 
@@ skipping 139 matching lines @@
               usageException(
                   'Could not find a command named "${argResults.rest[0]}".');
             }
 
             command.usageException('Could not find a subcommand named '
                 '"${argResults.rest[0]}" for "$commandString".');
           }
         }
 
         // Step into the command.
-        var parent = command;
         argResults = argResults.command;
         command = commands[argResults.name];
         command._globalResults = topLevelResults;
         command._argResults = argResults;
         commands = command._subcommands;
         commandString += " ${argResults.name}";
 
         if (argResults['help']) {
           command.printUsage();
           return new Future.value();
@@ skipping 205 matching lines @@
   var buffer = new StringBuffer(
       'Available ${isSubcommand ? "sub" : ""}commands:');
   for (var name in names) {
     buffer.writeln();
     buffer.write('  ${padRight(name, length)}   '
         '${commands[name].description.split("\n").first}');
   }
 
   return buffer.toString();
 }