Access the `.next` method only once as per https://github.com/tc39/ecma262/pull/988

[Jamesernator]

The tests for this seem to be already merged into Test262 (here and here) so the spec text here should probably also be changed to reflect the changed behavior.

[domenic]

Yep, that's the plan, just haven't had the time yet. It's a bit tricky since a lot of the underlying spec machinery has changed out from under this proposal.