The nice thing about this is that it should work fine with "vanilla" uses.
The "vanilla" callback is called before doing anything deferred-specific (if it's provided, that is), and 'fs' async methods don't return anything, so it's not breaking anything, unless there's some weird code out there that depends on 'fs' aync methods to throw an error when they're called without a callback or return undefined always.

Because of return value hiding, I disabled setTimeout.