Type ReactElementSymbol (#7564)

(cherry picked from commit a72a156f58)

src/shared/utils/ReactElementSymbol.js

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactElementSymbol
+ * @flow
  */
 
 'use strict';