fix eslint violations in Object.assign polyfill

app/assets/javascripts/extensions/object.js.es6

-/* eslint-disable */
+/* eslint-disable no-restricted-syntax */
 
-// Taken from https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill
-if (typeof Object.assign != 'function') {
-  Object.assign = function (target, varArgs) { // .length of function is 2
-    'use strict';
+// Adapted from https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign#Polyfill
+if (typeof Object.assign !== 'function') {
+  Object.assign = function assign(target, ...args) {
     if (target == null) { // TypeError if undefined or null
       throw new TypeError('Cannot convert undefined or null to object');
     }
 
-    var to = Object(target);
+    const to = Object(target);
 
-    for (var index = 1; index < arguments.length; index++) {
-      var nextSource = arguments[index];
+    for (let index = 0; index < args.length; index += 1) {
+      const nextSource = args[index];
 
       if (nextSource != null) { // Skip over if undefined or null
-        for (var nextKey in nextSource) {
+        for (const nextKey in nextSource) {
           // Avoid bugs when hasOwnProperty is shadowed
           if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
             to[nextKey] = nextSource[nextKey];