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

@ricea commented on this pull request.



> +<div algorithm="ReadableStream transfer steps">
+{{ReadableStream}} objects are [=transferable objects=]. Their [=transfer steps=], given |value| and
+|dataHolder|, are:
+
+ 1. [=Create a new MessagePort object=] whose [=owner=] is the [=incumbent settings object=] and let
+    |port1| be that object.
+ 1. [=Create a new MessagePort object=] whose [=owner=] is the [=incumbent settings object=] and let
+    |port2| be that object.
+ 1. [=Entangle=] the |port1| and |port2| objects.
+ 1. If ! [$IsReadableStreamLocked$](|value|) is true, throw a {{TypeError}} exception.
+ 1. Let |writable| be a [=new=] {{WritableStream}}.
+ 1. Perform ! [$SetUpCrossRealmTransformWritable$](|writable|, |port1|).
+ 1. Let |promise| be ! [$ReadableStreamPipeTo$](|value|, |writable|, false, false, false, undefined).
+ 1. Set |promise|.\[[PromiseIsHandled]] to true.
+ 1. Let |messagePortDataHolder| be { \[[Type]]: `"MessagePort"` }.
+ 1. Perform {{MessagePort}}'s transfer steps <!-- TODO(ricea): Link this somehow --> with |port2|

Done. I made it slightly higher-level to avoid poking around in the internals of the serialization algorithm.

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