Invariants: Prototype of (async) iterators

[Jack Works]

Description

Iterators/generators should have %IteratorPrototype% as its prototype, or in its prototype chain before any ECMAScript code runs.

Async iterators/generators should have %AsyncIteratorPrototype% as its prototype, or in its prototype chain before any ECMAScript code runs.

Violations?

At the current point, there is nothing violating this in the 262 spec.

Rationale: Why did we have this invariant?

Make Iterator Helpers work for all built-in iterators. (initial discussion tc39/ecma262#2045 (comment))

[jmdyck]

Iterators/generators should have %IteratorPrototype% as its prototype, or in its prototype chain before any ECMAScript code runs.

• Users are free to create iterators and set the prototype to pretty much anything they like, so this can presumably only be a constraint on "built-in" iterators (i.e., iterators created by built-in functions).

• An iterator doesn't exist before any ECMAScript code runs, so it doesn't make sense to talk about its prototype chain before any ECMAScript code runs.

So I think a better way to express this invariant would be:

• When a built-in function creates an iterator, it should specify its prototype to be either %IteratorPrototype% or some %FooIteratorPrototype%.

• Every %FooIteratorPrototype% should have %IteratorPrototype% on its prototype chain when it's created.