Re: [whatwg/webidl] Add `async iterable<T>` type to WebIDL (PR #1397)

@lucacasonato commented on this pull request.



> +<div id="async-iterable-to-js" algorithm="convert an async iterable to a JavaScript value">
+    An IDL <a lt="async iterable type">async iterable&lt;|<var ignore>T</var>&gt;</a> value is
+    [=converted to a JavaScript value|converted=] to a JavaScript object as follows:
+
+    1.  Return the JavaScript object that represents the same async iterable as the IDL value.
+</div>

> Would it be an option for us to just disallow converting an IDL async iterable type back to a JavaScript value?

That was indeed my intention, except for the case where the value just has to pass through IDL momentarially (like in functions like `setTimeout`, that call a passed callback with passed arguments). I am not sure now whether this is needed, but earlier reading of the spec made me beleive that all IDL values had to have a way to convert back to JS for these cases - maybe I am wrong.

> I think we should make "convert from a JavaScript object" not call GetIterator():

That seems reasonable. I think the converter should do the following:

1. Check `Type(V)` is `Object`
2. Check that `V` has a `Symbol.asyncIterator` or `Symbol.iterator` property, and if so, store it. (But do not call it yet.)

Then the `open an async iterable` algorithm:

1. Calls the iterator method stored during the conversion (using [ECMA262's `GetIteratorFromMethod`](https://tc39.es/ecma262/#sec-getiteratorfrommethod))
2. Returns the ECMA262's `Iterator` record (or an IDL wrapper thereof)

The other async iterable algorithms are then modified to work with it.

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