Add a CommandRunner class for dispatching commands to args.

pkg/args/lib/src/utils.dart

Index: pkg/args/lib/src/utils.dart
diff --git a/pkg/collection/lib/src/utils.dart b/pkg/args/lib/src/utils.dart
similarity index 56%
copy from pkg/collection/lib/src/utils.dart
copy to pkg/args/lib/src/utils.dart
index c9c753777f07c1c5bd930fa4a3a7e32a5377db0f..dc355dfb63bef320a989c69e259859acf22352fd 100644
--- a/pkg/collection/lib/src/utils.dart
+++ b/pkg/args/lib/src/utils.dart
@@ -2,12 +2,8 @@
 // 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 dart.pkg.collection.utils;
+library args.utils;
 
-/// A pair of values.
-class Pair<E, F> {
-  E first;
-  F last;
-
-  Pair(this.first, this.last);
-}
+/// Pads [source] to [length] by adding spaces at the end.
+String padRight(String source, int length) =>
+    source + ' ' * (length - source.length);