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In practice, every single "setter" in the platform is also a "creator" and vice versa. All four of them (DOMStringMap, HTMLOptionsCollection, HTMLSelectElement, and Storage). This would allow some simplifications to [[DefineOwnProperty]] and the [[Set]] that we need to define in bug 26521.
https://github.com/heycam/webidl/commit/cea5e80bddb828b377c6378d239654e1b13024f7