Add a CommandRunner class for dispatching commands to args.

pkg/args/test/command_parse_test.dart

Index: pkg/args/test/command_parse_test.dart
diff --git a/pkg/args/test/command_test.dart b/pkg/args/test/command_parse_test.dart
similarity index 99%
copy from pkg/args/test/command_test.dart
copy to pkg/args/test/command_parse_test.dart
index 1380e160087914b36198eb1a5cbdef90caea531b..ac530564b29b93fb945b131f08b9fb6ec40bf945 100644
--- a/pkg/args/test/command_test.dart
+++ b/pkg/args/test/command_parse_test.dart
@@ -2,7 +2,7 @@
 // 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';