Issue 4090 - Don't use let statements in compat.js, unsupported in older Chrome versions

lib/compat.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 framework stuff
 //
 
 function require(module)
 {
-  let result = require.scopes[module];
+  var result = require.scopes[module];
   if (typeof result == "function" && !(module in require.loaded))
   {
     result = require.scopes[module] = result();
     require.loaded[module] = true;
   }
   return result;
 }
 require.loaded = Object.create(null);
 require.scopes = Object.create(null);
 
@@ skipping 228 matching lines @@
   status: -1,
   notificationCallbacks: {},
   loadFlags: 0,
   INHIBIT_CACHING: 0,
   VALIDATE_ALWAYS: 0,
   QueryInterface: function()
   {
     return this;
   }
 };