Issue 5938 - Use top-level var for global variables

ext/common.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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/>.
  */
 
 "use strict";
 
+var ext = {};
+
 {
-  window.ext = {};
-
   let EventTarget = ext._EventTarget = function()
   {
     this._listeners = new Set();
   };
   EventTarget.prototype = {
     addListener(listener)
     {
       this._listeners.add(listener);
     },
     removeListener(listener)
     {
       this._listeners.delete(listener);
     },
     _dispatch(...args)
     {
       let results = [];
 
       for (let listener of this._listeners)
         results.push(listener(...args));
 
       return results;
     }
   };
 
 
   /* Message passing */
 
   ext.onMessage = new ext._EventTarget();
 }