Re: [ServiceWorker] ExtendableMessageEvent.source cannot be SameObject (#816)

That part of the IDL spec is just bogus, imo; there are existing IDL issues on that.  Any type that's always an object, including unions of interface types, Promise, etc, should be OK to use with `[SameObject]`.

Arguably, even a union for which _some_ types are an object should be OK with `[SameObject]`.

@heycam may have pointers to the IDL spec issues off the top of his head.

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