Arr.cloneArray is sort of mix of Array.map() and the coming soon Array.from() (or Array.slice with 1 argument). 
I would personally deprecate them. And in order to avoid calling deprecated functions, each call site has been replaced. 

I know that actually, we would have rewritten the call sites twice, but the second time a sed script could simply do it (replacing each time we match Array.prototype.slice.call).

Is that bothering you ?

Florent