Add a CommandRunner class for dispatching commands to args.

pkg/args/lib/src/help_command.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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 pub.command.help;
+library args.help_command;
 
-import 'dart:async';
+import '../command_runner.dart';
 
-import '../command.dart';
+/// The built-in help command that's added to every [CommandRunner].
+///
+/// This command displays help information for the various subcommands.
+class HelpCommand extends Command {
+  final name = "help";
+  String get description =>
+      "Display help information for ${runner.executableName}.";
+  String get usage => "${runner.executableName} help [command]";
 
-/// Handles the `help` pub command.
-class HelpCommand extends PubCommand {
-  String get description => "Display help information for Pub.";
-  String get usage => "pub help [command]";
-  bool get takesArguments => true;
-
-  Future onRun() {
+  void run() {
     // Show the default help if no command was specified.
-    if (commandOptions.rest.isEmpty) {
-      PubCommand.printGlobalUsage();
-      return null;
+    if (options.rest.isEmpty) {
+      runner.printUsage();
+      return;
     }
 
     // Walk the command tree to show help for the selected command or
     // subcommand.
-    var commands = PubCommand.mainCommands;
+    var commands = runner.topLevelCommands;
     var command = null;
-    var commandString = "pub";
+    var commandString = runner.executableName;
 
-    for (var name in commandOptions.rest) {
+    for (var name in options.rest) {
       if (commands.isEmpty) {
         command.usageError(
             'Command "$commandString" does not expect a subcommand.');
       }
 
       if (commands[name] == null) {
         if (command == null) {
-          PubCommand.usageErrorWithCommands(
-              commands,
-              'Could not find a command named "$name".');
+          runner.usageError('Could not find a command named "$name".');
         }
 
         command.usageError(
             'Could not find a subcommand named "$name" for "$commandString".');
       }
 
       command = commands[name];
       commands = command.subcommands;
       commandString += " $name";
     }
 
     command.printUsage();
-    return null;
   }
 }