Add a CommandRunner class for dispatching commands to args.

pkg/args/lib/src/usage_exception.dart

Index: pkg/args/lib/src/usage_exception.dart
diff --git a/pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart b/pkg/args/lib/src/usage_exception.dart
similarity index 51%
copy from pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart
copy to pkg/args/lib/src/usage_exception.dart
index 1b3d037c5bce3add4829e93b527bc5eb7f01f821..35cc7442e549b93da0e825ae660171c7361c3647 100644
--- a/pkg/docgen/test/multi_library_code/lib/test_lib2_foo.dart
+++ b/pkg/args/lib/src/usage_exception.dart
@@ -2,14 +2,13 @@
 // 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);
 
-}
-
-main() {
-  print('hello world');
+  String toString() => "$message\n\n$usage";
 }