Issue 4795 - Use modern JavaScript syntax

lib/prefs.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 /** @module prefs */
 
 "use strict";
 
-let {EventEmitter} = require("events");
+const {EventEmitter} = require("events");
 
 const keyPrefix = "pref:";
 
 let eventEmitter = new EventEmitter();
 let overrides = Object.create(null);
 
 /** @lends module:prefs.Prefs */
 let defaults = Object.create(null);
 
 /**
@@ skipping 278 matching lines @@
           eventEmitter.emit(pref);
         }
       }
     });
   }
 
   Prefs.untilLoaded = Promise.all([localLoaded, managedLoaded]).then(onLoaded);
 }
 
 init();