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

@ricea commented on this pull request.



> + 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.
+  1. Let |type| be ? [$Get$](|data|, `"type"`).
+  1. Let |value| be ? [$Get$](|data|, `"value"`).
+  1. If ! [$Type$](|type|) is not String, return.
+  1. If |type| is `"pull"`,
+   1. If |backpressurePromise| is not undefined,
+    1. [=Resolve=] |backpressurePromise| with undefined.
+    1. Set |backpressurePromise| to undefined.
+  1. Otherwise, if |type| is `"error"`,
+   1. Perform ! [$WritableStreamDefaultControllerErrorIfNeeded$](|controller|, |value|).
+   1. If |backpressurePromise| is not undefined,
+    1. [=Resolve=] |backpressurePromise| with undefined.
+    1. Set |backpressurePromise| to undefined.

I'm leaning towards "no" now, since in platonic spec-land it is redundant to disentangle from both sides, and in practice implementations need to implement MessagePort `close()` in such a way that both sides get closed after all queued messages have been delivered.

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