Use more ES2015.

_baseMerge.js

@@ -20,13 +20,13 @@ function baseMerge(object, source, srcIndex, customizer, stack) {
   if (object === source) {
     return;
   }
-  baseFor(source, function(srcValue, key) {
+  baseFor(source, (srcValue, key) => {
     if (isObject(srcValue)) {
       stack || (stack = new Stack);
       baseMergeDeep(object, source, key, srcIndex, baseMerge, customizer, stack);
     }
     else {
-      var newValue = customizer
+      const newValue = customizer
         ? customizer(object[key], srcValue, (key + ''), object, source, stack)
         : undefined;