Re: [heycam/webidl] undefined can't be a member of a union type (#962)

I had totally missed that writable attributes can take undefined in a union.

What would be the use for undefined being nullable? I don't think that would make a lot of sense outside of unions.

I like `T?` being shorthand for `T or null or undefined` (sidenote: there is no `null` type), but if that's a real union, wouldn't that make it impossible to distinguish `null` and `undefined` in a union? `T? or undefined` would be invalid, and existing APIs would expect the current conversion from ES `null`or `undefined` to IDL `null`, so it seems like we wouldn't be able to fix it on that level.

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