Type ReactCurrentOwner (#7648)

Simpler than ReactOwner :)
(cherry picked from commit 2559030c34)

src/isomorphic/classic/element/ReactCurrentOwner.js

@@ -7,10 +7,13 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactCurrentOwner
+ * @flow
  */
 
 'use strict';
 
+import type { ReactInstance } from 'ReactInstanceType';
+
 /**
  * Keeps track of the current owner.
  *
@@ -23,7 +26,7 @@ var ReactCurrentOwner = {
    * @internal
    * @type {ReactComponent}
    */
-  current: null,
+  current: (null: null | ReactInstance),
 
 };
 

src/renderers/shared/stack/reconciler/ReactInstanceType.js

@@ -34,4 +34,5 @@ export type ReactInstance = {
   _mountImage: any,
   // __DEV__
   _debugID: DebugID,
+  _warnedAboutRefsInRender: bool,
 };