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

@ricea commented on this pull request.



> +Transferable streams are implemented using a special kind of identity transform which has the
+[=writable side=] in one realm and the [=readable side=] in another realm. The following abstract
+operations are used to implement these "cross-realm transforms".
+
+<div algorithm>
+ <dfn abstract-op lt="PackAndPostMessage">PackAndPostMessage(|port|, |type|, |value|)</dfn> performs
+ the following steps:
+
+ 1. Let |message| be [$OrdinaryObjectCreate$](null).
+ 1. Perform [$CreateDataProperty$](|message|, `"type"`, |type|).
+ 1. Perform [$CreateDataProperty$](|message|, `"value"`, |value|).
+ 1. Let |targetPort| be the port with which |port| is entangled, if any; otherwise let it be null.
+ 1. Let |options| be «[ `"transfer"` → « » ]».
+ 1. Run the [=message port post message steps=] providing |targetPort|, |message| and |options|.
+
+ <p class="note">A JavaScript object is used for transfer to avoid having to duplicate the [=message

The duplication is not bad in spec-land, but it's a can of worms in implementation land.

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