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Consider an interface with a named creator and [OverrideBuiltins] (in fact, the only interface with a named creator in the platform has that extended attribute). When a set happens for a currently-nonexistent, per current spec [[GetOwnProperty]] will return undefined. Then the ES impl will go up the proto chain looking for a setter, and if found call it. That doesn't seem correct; we actually want to call the named creator in this case. We should probably just override [[Put]] when we have a named creator. And maybe transition all this stuff to the ES6 MOP in the process...
Fixed by https://github.com/heycam/webidl/commit/646bd2ae909117329d8db89dd20f39bba3e0a513