fix stack overflow in Flow_polymorphic_ast_mapper
fix stack overflow in Flow_polymorphic_ast_mapper

Summary:
objects with a ton of properties would cause Flow_polymorphic_ast_mapper to stack overflow because stdlib's List.map is not tail recursive. `Base.List.map` is, though.

Changelog: [fix] Fix a crash when objects have a very large number of properties

Reviewed By: SamChou19815

Differential Revision: D36478701

fbshipit-source-id: 849afd12050c4f7471eed4e14edde06cf6f45436

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