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Per bug 23212 and http://heycam.github.io/webidl/#es-iterators it appears that if you define an object that has indexed property getters and a length, you get Symbol.iterator for free. Given that all "legacy" objects that would need legacyiterable are of this form, it seems legacyiterable is in fact not needed.
https://github.com/heycam/webidl/pull/68
https://github.com/heycam/webidl/commit/0adc47199a851678686e89c3b1d0f5573c560176