Add command support to args.

pkg/args/test/args_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 args_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:args/args.dart';
 
 main() {
@@ skipping 52 matching lines @@
   group('ArgParser.getDefault()', () {
     test('returns the default value for an option', () {
       var parser = new ArgParser();
       parser.addOption('mode', defaultsTo: 'debug');
       expect(parser.getDefault('mode'), 'debug');
     });
 
     test('throws if the option is unknown', () {
       var parser = new ArgParser();
       parser.addOption('mode', defaultsTo: 'debug');
-      expect(()=>parser.getDefault('undefined'),
-          throwsArgumentError);
+      throwsIllegalArg(() => parser.getDefault('undefined'));
     });
   });
 
   group('ArgResults.options', () {
     test('returns the provided options', () {
       var parser = new ArgParser();
       parser.addFlag('woof');
       parser.addOption('meow');
       var args = parser.parse(['--woof', '--meow', 'kitty']);
       expect(args.options, hasLength(2));
@@ skipping 21 matching lines @@
   });
 }
 
 throwsIllegalArg(function) {
   expect(function, throwsArgumentError);
 }
 
 throwsFormat(ArgParser parser, List<String> args) {
   expect(() => parser.parse(args), throwsFormatException);
 }