Global iterator type is coming: https://github.com/tc39/proposal-iterator-helpers

But a method named `groupBy` on iterators traditionally means a different thing: https://github.com/tc39/proposal-array-grouping/issues/51#is...

Global iterable type it's too late for, since there's many extant iterables in the language and on the web which don't have it in their prototype chain and can't reasonably be changed.