Axure, another design and prototyping tool, uses a Subversion-style model, where you have both a cloud and local copy. You pull down the initial file, save it locally, then check out individual pages or components to work on before pushing them back up.

It means you have to manually manage the commit state which is a bit of a pain, but it’s good because you still have the file and can work on it if you can’t access the cloud for whatever reason.