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.

It's my understanding that we only need to disentangle the port on one side. Although from the implementers' perspective it might be better to explicitly disentangle on both sides. Let me think about it.

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