Get rid of unneeded Future.immediate() calls.

Future.then() will now implicitly wrap the callback result in a
future if it isn't one, so this manual wrapping can be removed.

Also take advantage of the fact that Future.immediate() is always
async now and remove some workarounds.

Finally, noticed there were two copies of the git-related code
floating around so killed the old one.

Review URL: https://codereview.chromium.org//12042053

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@17519 260f80e4-7a28-3924-810f-c04153c831b5