Normative: close underlying iterators when return is called before next

[bakkot]

Fixes #266 (cc @syg).

Note that we'll need consensus from committee before merging.

(The slot has to be named [[UnderlyingIterator]] instead of [[Iterated]] because we use [[Iterated]] for the name of a slot on %WrapForValidIteratorPrototype% objects, and we don't want to accidentally make iterator helper objects valid .call targets for %WrapForValidIteratorPrototype%.next.)

[ljharb]

Any chance you could provide a test case that fails without this PR?

[bakkot]

Something along the lines of

let returnCalls = 0;
let it = {
  __proto__: Iterator.prototype,
  next() {},
  return() {
    ++returnCalls;
  },
};
let mapped = it.map(() => {});
mapped.return();
assert(returnCalls === 1); // would be 0 without this PR
mapped.return();
assert(returnCalls === 1); // only gets called the first time

[ljharb]

I've implemented and tested the changes in my polyfill, works as advertised.