Issue 4795 - Use modern JavaScript syntax

composer.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/>.
  */
+
+"use strict";
 
 let targetPageId = null;
 
 function onKeyDown(event)
 {
   if (event.keyCode == 27)
   {
     event.preventDefault();
     closeDialog();
   }
@@ skipping 60 matching lines @@
         break;
       case "composer.dialog.close":
         window.close();
         break;
     }
   });
 
   window.removeEventListener("load", init);
 }
 window.addEventListener("load", init, false);