Fix flow errors

Reviewed By: gabelevi

Differential Revision: D2796260

fb-gh-sync-id: 8425e57a313326afb6f6ae5bb8b86a569f462b06

Libraries/Image/ImageSource.js

@@ -11,6 +11,6 @@
  */
 'use strict';
 
-type ImageSource = {
+export type ImageSource = {
   uri: string;
 };