Add a CommandRunner class for dispatching commands to args.

pkg/args/test/command_parse_test.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_test;
+library command_parse_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 import 'utils.dart';
 
 void main() {
   group('ArgParser.addCommand()', () {
     test('creates a new ArgParser if none is given', () {
       var parser = new ArgParser();
       var command = parser.addCommand('install');
@@ skipping 182 matching lines @@
 
       var args = parser.parse(['cmd', 'subcmd', 'other', 'stuff']);
       expect(args.command.name, equals('cmd'));
       expect(args.rest, isEmpty);
       expect(args.command.command.name, equals('subcmd'));
       expect(args.command.rest, isEmpty);
       expect(args.command.command.rest, equals(['other', 'stuff']));
     });
   });
 }