Spec: AsyncFunctionStart does not accept closure argument

[js-choi]

Step 4 of the specification currently says:

Perform AsyncFunctionStart(promiseCapability, fromAsyncClosure).

This is an error. AsyncFunctionStart, which in turn calls AsyncBlockStart, expects its second argument to be a Parse Node.

We might want first to call CreateBuiltinFunction on fromAsyncClosure or something.

This issue was discovered by Aditi Singh of Igalia.

[bakkot]

You can change AsyncBlockStart to accept either a parse node or an Abstract Closure with no parameters. That's what GeneratorStart does.