[hack] Set.forEach callback can return mixed instead of void
[hack] Set.forEach callback can return mixed instead of void

Summary: This is for consistency with Map.forEach, and also is convenient (I noticed this while reviewing FlowFixMe for www when enabling Flow checks for its polyfills)

Reviewed By: jeffmo

Differential Revision: D2982421

fb-gh-sync-id: 7d4eab32ea43c5cc812c9eb7fc7a749bbf221658
shipit-source-id: 7d4eab32ea43c5cc812c9eb7fc7a749bbf221658

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