Apply even more let/const transforms.

groupBy.js

@@ -2,10 +2,10 @@ import baseAssignValue from './_baseAssignValue.js';
 import createAggregator from './_createAggregator.js';
 
 /** Used for built-in method references. */
-var objectProto = Object.prototype;
+const objectProto = Object.prototype;
 
 /** Used to check objects for own properties. */
-var hasOwnProperty = objectProto.hasOwnProperty;
+const hasOwnProperty = objectProto.hasOwnProperty;
 
 /**
  * Creates an object composed of keys generated from the results of running
@@ -30,7 +30,7 @@ var hasOwnProperty = objectProto.hasOwnProperty;
  * _.groupBy(['one', 'two', 'three'], 'length');
  * // => { '3': ['one', 'two'], '5': ['three'] }
  */
-var groupBy = createAggregator((result, value, key) => {
+const groupBy = createAggregator((result, value, key) => {
   if (hasOwnProperty.call(result, key)) {
     result[key].push(value);
   } else {