Add CodeOutput to prepare for emission without in-memory buffers.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//812233004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42573 260f80e4-7a28-3924-810f-c04153c831b5