Re: [heycam/webidl] Named properties object / named property visibility algorithm with property on Window.prototype (#607)

Ah, @Ms2ger clarified that he's talking about this case, specifically:

1. There is an element with id "foo".
2. There is a property on Window.prototype with name "foo".
3. You do a property lookup for "foo", via [[Get]] or [[GetOwnProperty]] on the named properties object.

Per spec, those lookups should not find the element, because https://heycam.github.io/webidl/#named-properties-object-getownproperty step 2 does the visibility check on the _window_, not on the named properties object itself, finds the property on Window.prototype, and then says the named property object has no property named "foo".

As @Ms2ger points out, no browsers do that.  What's really at stake here is making sure that the named properties object doesn't shadow anything higher on the prototype chain, so that's what Firefox checks: that there is no such property on the prototype chain of the named properties object.  Window.prototype is not on said proto chain, but also can't be shadowed by the named properties object, so doesn't have to be checked.

I suspect we could take out step 2 there and remove the check for a "named properties object" in https://heycam.github.io/webidl/#dfn-named-property-visibility step 5.1 and things would be just fine web compat wise, and the behavior simpler/saner.
    

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