Issue 4795 - Use modern JavaScript syntax

utils.js

 "use strict";
 
-let backgroundPage = ext.backgroundPage.getWindow();
-let require = backgroundPage.require;
+const {require, Services} = ext.backgroundPage.getWindow();
 
-let Services = backgroundPage.Services;
-let {Synchronizer} = require("synchronizer");
-let {Utils} = require("utils");
-let {Prefs} = require("prefs");
-let {FilterStorage} = require("filterStorage");
-let {FilterNotifier} = require("filterNotifier");
+const {Synchronizer} = require("synchronizer");
+const {Utils} = require("utils");
+const {Prefs} = require("prefs");
+const {FilterStorage} = require("filterStorage");
+const {FilterNotifier} = require("filterNotifier");
 
-let {Subscription, DownloadableSubscription} = require("subscriptionClasses");
-let {Filter, BlockingFilter} = require("filterClasses");
-let {defaultMatcher} = require("matcher");
+const {Subscription, DownloadableSubscription} = require("subscriptionClasses");
+const {Filter, BlockingFilter} = require("filterClasses");
+const {defaultMatcher} = require("matcher");
 
 /**
  * Shortcut for document.getElementById(id)
  */
 function E(id)
 {
   return document.getElementById(id);
 }