Make arg parser return growable list of rest arguments.

This allows the user to remove them when he's done, which is what
is currently being done.

Review URL: https://codereview.chromium.org//12316155

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@19119 260f80e4-7a28-3924-810f-c04153c831b5