R1:91eb751d6f3d

Use `k in map`, not `array.indexOf`, in Event.js

Summary: See T5023.

Test Plan: Dragged stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5023

Differential Revision: https://secure.phabricator.com/D9082