Wrapping expression to be awaited into Future.value

Not sure if using raw name in `make.identifier('Future.value’)` is OK.
Otherwise, trivial change.
I don't want to commit the changes to this file.  Can you remove it from the PR?

I honestly don't know what features of barback we rely on, but I'm leery of allowing very old versions --- I also don't know how we'll test that we continue to work with old versions.  The one on pub is 0.15.2+2.