Issue 2255 - Use "for (.. of ..)" loop for compatibility with ES6 generators

lib/io.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2015 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 200 matching lines @@
         let lineBreak = this.lineBreak;
 
         function writeChunk()
         {
           let array = encoder.encode(buf.join(lineBreak) + lineBreak);
           buf = [];
           bufLen = 0;
           return f.write(array);
         }
 
-        for (let line in data)
+        for (let line of data)
         {
           buf.push(line);
           bufLen += line.length;
           if (bufLen >= BUFFER_SIZE)
             yield writeChunk();
         }
 
         if (bufLen)
           yield writeChunk();
 
@@ skipping 91 matching lines @@
         else
           callback(e);
       });
     }
     catch(e)
     {
       callback(e);
     }
   }
 }