Update downloadable.js

Changed let to var. 
Added 'use strict';

Author: VladimirZaets

app/code/Magento/Downloadable/view/frontend/web/js/downloadable.js

@@ -10,7 +10,8 @@ define([
     'jquery-ui-modules/widget',
     'Magento_Catalog/js/price-box'
 ], function ($) {
-
+    'use strict';
+    
     /**
      * Downloadable widget
      */
@@ -78,7 +79,7 @@ define([
          * @private
          */
         reloadAllCheckText: function () {
-            let allChecked = true,
+            var allChecked = true,
                 allElementsCheck = $(this.options.allElements),
                 allElementsLabel = $('label[for="' + allElementsCheck.attr('id') + '"] > span');