Re-enable writable filesystem federation

* Fixes https://www.pivotaltracker.com/story/show/70954532
* Depends on https://issues.jboss.org/browse/MODE-2252
Have you run into a scenario where the doc is null?

Minor: please format the brackets at 117 in the style of the 1TBS variant of K&R (http://en.wikipedia.org/wiki/Indent_style#Variant:_1TBS)
---
if () {
    return doc;
}