ContextOptimizer optimizes unions
ContextOptimizer optimizes unions

Summary: This diff ensures that all unions are optimized before being exported.

Reviewed By: samwgoldman

Differential Revision: D23619751

fbshipit-source-id: e68bf4650d4ec3f7e358528f55b0440bdf12ee6c

facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow