Summary:
Update mapWithSeparator so that Flow can reason about the arguments and return
type. For simplicity, it is expected that the type of the separator will be the
same as that of the mapped item.
Reviewed By: vjeux
Differential Revision: D3323557
fbshipit-source-id: 75b59e928d4e8c309b5933499a14744370ee5660
Sean Kinsey