Issue 4795 - Use modern JavaScript syntax

lib/filterValidation.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 filterValidation */
 
-let {Filter, InvalidFilter, ElemHideBase} = require("filterClasses");
-let {Utils} = require("utils");
+"use strict";
+
+const {Filter, InvalidFilter, ElemHideBase} = require("filterClasses");
+const {Utils} = require("utils");
 
 /**
  * An error returned by
  * {@link module:filterValidation.parseFilter parseFilter()} or
  * {@link module:filterValidation.parseFilters parseFilters()}
  * indicating that a given filter cannot be parsed,
  * contains an invalid CSS selector or is a filter list header.
  *
  * @constructor
  */
@@ skipping 24 matching lines @@
    *
    * @type {?number}
    */
   lineno: null,
 
   /**
    * Returns a detailed translated error message.
    *
    * @return {string}
    */
-  toString: function()
+  toString()
   {
     let message;
     if (this.reason)
       message = Utils.getString(this.reason);
     else
       message = ext.i18n.getMessage(
         this.type.replace(/-/g, "_"),
         "selector" in this ? "'" + this.selector + "'" : null
       );
 
@@ skipping 87 matching lines @@
 
     if (error)
     {
       error.lineno = i + 1;
       errors.push(error);
     }
   }
 
   return {filters: filters, errors: errors};
 };