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

@MattiasBuelens 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>

Hmm, maybe the real problem is that the IDL machinery should not be calling `GetIterator()` from within the conversion logic in the first place. 🤔 For example: if an IDL operation accepts *two* `async iterable<T>` values as parameters, and `GetIterator()` succeeds for the first value but fails for the second value (e.g. because it's a locked `ReadableStream`), then the operation errors immediately with a `TypeError` but the first value is now permanently "locked".

I think we should make "convert from a JavaScript object" *not* call `GetIterator()`:
* Make an IDL async iterable *only* represent a JavaScript object.
* Add a separate type for an "IDL async iterator", which is represented by a JavaScript `Iterator` record.
* Add an "open an async iterable" algorithm, which takes an IDL async iterable, calls `GetIterator()` and returns it as an IDL async iterator.
* Refactor the "get the next value of an async iterable" and "close an async iterable" algorithms to work on an IDL async iterator instead.

Thoughts?

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