[hack] Delegate to `Map.mem` for `MyMap.has_key`
[hack] Delegate to `Map.mem` for `MyMap.has_key`

Reviewed By: samwgoldman

Differential Revision: D18356789

fbshipit-source-id: 09be05a0eee7d9ba9217614b3d1a33a8b11a69aa

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow