Re: [heycam/webidl] Allow async iterators to specify return algorithms (#805)

domenic commented on this pull request.



> +        *   the platform object |object|,
+        *   the identifier "<code>return</code>", and
+        *   the type "<code>method</code>".
+
+        If this threw an exception |e|, then:
+        1.  Perform [=!=] [$Call$](|returnPromiseCapability|.\[[Reject]],
+            <emu-val>undefined</emu-val>, « |e| »).
+        1.  Return |returnPromiseCapability|.\[[Promise]].
+
+    1.  If |object| is not a [=default asynchronous iterator object=] for |interface|, then:
+        1.  Let |error| be a new {{ECMAScript/TypeError}}.
+        1.  Perform [=!=] [$Call$](|returnPromiseCapability|.\[[Reject]],
+            <emu-val>undefined</emu-val>, « |error| »).
+        1.  Return |returnPromiseCapability|.\[[Promise]].
+
+    1.  If |object|'s [=default asynchronous iterator object/ongoing promise=] is not undefined, then:

Mmm nice catch. Since we made it an IDL Promise, we should probably union it with null (another IDL value), not undefined (which is always `<emu-val>undefined</emu-val>`, a JS value). I switched all instances to do so.

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