Don't delay IOSink.close() with no subscription to the next instance.

This fixes the issue with pub I found. Added TODO in pub, as
stdout/stderr doesn't really need wrapping anymore.

BUG=

Review URL: https://codereview.chromium.org//14261003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@21431 260f80e4-7a28-3924-810f-c04153c831b5