fixes #868 (#1878)

Summary:
**Summary**
- Updated the DraftEntity flow type to be `string`. fixes #868

**Test Plan**
N/A
Pull Request resolved: #1878

Differential Revision: D10188002

fbshipit-source-id: e52da4d8dd708c7ed58e7d6f8c585e86835f1bb2

Author: juliankrispel

src/model/entity/DraftEntityType.js

@@ -13,6 +13,4 @@
 
 'use strict';
 
-import typeof ComposedEntityType from 'ComposedEntityType';
-
-export type DraftEntityType = $Keys<ComposedEntityType>;
+export type DraftEntityType = string;