Add a CommandRunner class for dispatching commands to args.

pkg/args/lib/src/help_command.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.help_command;
+
+import '../command_runner.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]";
+
+  void run() {
+    // Show the default help if no command was specified.
+    if (options.rest.isEmpty) {
+      runner.printUsage();
+      return;
+    }
+
+    // Walk the command tree to show help for the selected command or
+    // subcommand.
+    var commands = runner.commands;
+    var command = null;
+    var commandString = runner.executableName;
+
+    for (var name in options.rest) {
+      if (commands.isEmpty) {
+        command.usageException(
+            'Command "$commandString" does not expect a subcommand.');
+      }
+
+      if (commands[name] == null) {
+        if (command == null) {
+          runner.usageException('Could not find a command named "$name".');
+        }
+
+        command.usageException(
+            'Could not find a subcommand named "$name" for "$commandString".');
+      }
+
+      command = commands[name];
+      commands = command.subcommands;
+      commandString += " $name";
+    }
+
+    command.printUsage();
+  }
+}