You'd have to check which version of v8 added fat-arrow support... I've been using babeljs for most of my stuff regardless of node/v8 version as I want async/await support which is a ways off still.
The next+1 branch has a fairly recent version of v8 4.5 that has fat arrows and other new language features shipped. My guess is that next+1 will become 4.0.0.