fix: added FlowFixMe on Array.prototype.flatMap (#2131)

- Fix for flow-bin 0.96, which complains about flatMap not existing.

src/polyfills/flatMap.js

@@ -9,6 +9,7 @@ declare function flatMap<T, U>(
 // $FlowFixMe
 const flatMap = Array.prototype.flatMap
   ? function(list, fn) {
+      // $FlowFixMe
       return Array.prototype.flatMap.call(list, fn);
     }
   : function(list, fn) {