pkg/args: expose immutable collections

BUG= https://code.google.com/p/dart/issues/detail?id=10127
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//19810004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@25347 260f80e4-7a28-3924-810f-c04153c831b5