Re: [dom] Supported property names for NamedNodeMap (#141)

I think we would also need to add something that ensures these objects cannot be made non-extensible, as otherwise you could do `Object.preventExtensions(o)` and that would trigger the

>  If the object is non-extensible, the returned List must contain only the keys of all own properties of the object that are observable using [[GetOwnProperty]]. 

clause. (That clause is admittedly a bit ambiguous; it seems like it's trying to place an upper bound, but the phrasing "*all* own properties" seems to imply a lower bound too.)

It looks like making properties non-configurable is guarded against already in the platform object [[DefineOwnProperty]] definition.

If this is taken care of then I agree there is no invariant violation (just a slightly-annoying mismatch).

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