The proposal for adding flat and flatMap to Array.prototype has been amended to add the names to Array's list of unscopable names.
We're shipping flat and flatMap since 62, and they just advanced to stage 4, so we should add them to unscopables soon.
Pushed by firstname.lastname@example.org:
Add "flat" and "flatMap" to Array.prototype[@@unscopables]. r=jorendorff