Use more ES2015.

_baseFindKey.js

@@ -10,8 +10,8 @@
  * @returns {*} Returns the found element or its key, else `undefined`.
  */
 function baseFindKey(collection, predicate, eachFunc) {
-  var result;
-  eachFunc(collection, function(value, key, collection) {
+  let result;
+  eachFunc(collection, (value, key, collection) => {
     if (predicate(value, key, collection)) {
       result = key;
       return false;