Update pub to use the new command runner API in args.

sdk/lib/_internal/pub/lib/src/command/help.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 pub.command.help;
-
-import 'dart:async';
-
-import '../command.dart';
-
-/// 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() {
-    // Show the default help if no command was specified.
-    if (commandOptions.rest.isEmpty) {
-      PubCommand.printGlobalUsage();
-      return null;
-    }
-
-    // Walk the command tree to show help for the selected command or
-    // subcommand.
-    var commands = PubCommand.mainCommands;
-    var command = null;
-    var commandString = "pub";
-
-    for (var name in commandOptions.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".');
-        }
-
-        command.usageError(
-            'Could not find a subcommand named "$name" for "$commandString".');
-      }
-
-      command = commands[name];
-      commands = command.subcommands;
-      commandString += " $name";
-    }
-
-    command.printUsage();
-    return null;
-  }
-}