Fix flow

Author: eps1lon

packages/react-reconciler/src/ReactInternalTypes.js

@@ -124,7 +124,7 @@ export type Fiber = {
   // I'll avoid adding an owner field for prod and model that as functions.
   ref:
     | null
-    | (((handle: mixed) => void) & {_stringRef: ?string, ...})
+    | (((handle: mixed) => void) & {_stringRef?: string, _functionRef?: (handle: mixed) => void, ...})
     | RefObject,
 
   // Input is the data coming into process this fiber. Arguments. Props.