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

@ricea commented on this pull request.



> +    1. Set |backpressurePromise| to [=a new promise=].
+  1. Otherwise, if |type| is `"close"`,
+    1. Perform ! [$ReadableStreamDefaultControllerClose$](|controller|).
+    1. Disentangle |port|.
+  1. Otherwise, if |type| is `"error"`,
+   1. Perform ! [$ReadableStreamDefaultControllerError$](|controller|, |value|)
+   1. Disentangle |port|.
+ 1. Add a handler for |port|'s {{MessagePort/messageerror}} event with the following steps:
+  1. Let |error| be a new "{{DataCloneError}}" {{DOMException}}.
+  1. Perform ! [$CrossRealmTransformSendError$](|port|, |error|).
+  1. Perform ! [$ReadableStreamDefaultControllerError$](|controller|, |error|).
+  1. Disentangle |port|.
+ 1. Let |startAlgorithm| be an algorithm that returns undefined.
+ 1. Let |pullAlgorithm| be the following steps:
+  1. Perform ! [$PackAndPostMessage$](|port|, `"pull"`, undefined).
+  1. Return |backpressurePromise|.

I don't want to diverge too much from Blink's implementation, as until there's another implementation it's the only sanity check we have.

However, I can try this in Blink and see whether anything breaks. In principle the behaviour shouldn't change, and it would be a nice simplification to get rid of `backpressurePromise`.

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