The Iterator Helpers proposal defines 6 lazy methods that need to be implemented on %Iterator.prototype%:

• %Iterator.prototype%.map ( mapper )
• %Iterator.prototype%.filter ( filterer )
• %Iterator.prototype%.take ( limit )
• %Iterator.prototype%.drop ( limit )
• %Iterator.prototype%.asIndexedPairs ()
• %Iterator.prototype%.flatMap ( mapper )

The Iterator Helpers proposal defines 6 lazy synchronous iterator methods,
map, filter, take, drop, asIndexedPairs, and flatMap.
All of these methods are to return the newly defined builtin generators,
which we implement as self-hosted generators that are stored in an
internal slot and thus observably distinct from regular generators.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=c57d3e292e7dd08b97572908b66966c143abd030

https://hg.mozilla.org/mozilla-central/rev/cfad3fec4c86