Add a CommandRunner class for dispatching commands to args.

pkg/args/lib/src/usage_exception.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 test_lib2_foo;
+library args.usage_exception;
 
-export 'test_lib2_bar.dart';
+class UsageException implements Exception {
+  final String message;
+  final String usage;
 
-class Foo {
+  UsageException(this.message, this.usage);
 
+  String toString() => "$message\n\n$usage";
 }
-
-main() {
-  print('hello world');
-}