Re: [whatwg/streams] Support transferable streams (postMessage) (#1053)

@domenic commented on this pull request.



> +
+ <p class="note">As we are already in an errored state when this abstract operation is performed, we
+ cannot handle further errors, so we just discard them.</p>
+
+</div>
+
+<div algorithm>
+ <dfn abstract-op lt="SetUpCrossRealmTransformReadable">SetUpCrossRealmTransformReadable(|stream|,
+ |port|)</dfn> performs the following steps:
+
+ 1. Perform ! [$InitializeReadableStream$](|stream|).
+ 1. Let |controller| be a [=new=] {{ReadableStreamDefaultController}}.
+ 1. Let |backpressurePromise| be [=a new promise=].
+ 1. Add a handler for |port|'s {{MessagePort/message}} event with the following steps:
+  1. Let |data| be the data of the message.
+  1. If ! [$Type$](|data|) is not Object, return.

But there's no way for the situation to occur, per spec, so an assert is pretty much definitionally appropriate. We use them for cases like this all over the place (especially in the JS spec).

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