Update flat and flatMap to non-experimental (#3373)
Update flat and flatMap to non-experimental (#3373)

see https://github.com/tc39/proposal-flatMap and https://github.com/tc39/proposals/blob/master/finished-proposals.md as confirmation.
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data