Issue 6424 - Use internal symbol in ext namespace

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
@@ skipping 15 matching lines @@
 
   defineNamespace(ext, internal);
 
   let EventTarget = ext[internal].EventTarget = function()
   {
     defineNamespace(this, internal);
 
     this[internal].listeners = new Set();
   };
   EventTarget.prototype = {
+    [internal]: {
+      dispatch(...args)
+      {
+        return [...this[internal].listeners].map(listener => listener(...args));
+      }
+    },
     addListener(listener)
     {
       this[internal].listeners.add(listener);
     },
     removeListener(listener)
     {
       this[internal].listeners.delete(listener);
     }
   };
 
-  ext[internal].dispatchEvent = (eventTarget, ...args) =>
-                                [...eventTarget[internal].listeners].map(
-                                  listener => listener(...args)
-                                );
-
 
   /* Message passing */
 
   ext.onMessage = new ext[internal].EventTarget();
 }