error when mangle arrowfunctions

[fishbar]

var ls = (err, data) => {
  data += 1;
  if (err) {
    log.error(err);
    return err;
  }
  result = data;
  return data;
};

console.log(ls(null, 12));

lib/pass/remove-wasted-blocks.js should take care of arrowFunction expression