Refactor command code and add support for --help and --version.

utils/pub/command_version.dart

Index: utils/pub/command_version.dart
diff --git a/utils/pub/command_version.dart b/utils/pub/command_version.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a65a5ba84ac257bbc48f1106a4b08577bb97217a
--- /dev/null
+++ b/utils/pub/command_version.dart
@@ -0,0 +1,10 @@
+// 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.
+
+/** Handles the `version` pub command. */
+class VersionCommand extends PubCommand {
+  String get description() => 'print Pub version';
+
+  void onRun() => printVersion();
+}