code: function eval$exec(params) {
    var $exec = params.$exec;
    var $method = params.$method;
    var executor = this.js._executors[$exec];
    if (!executor) throw new Error('unknown executor ' + $exec);
    if ($method) {
      if (typeof executor[$method] != 'function')
        throw new Error('unknown method ' + $method + ' of executor ' + $exec);
      return util.toPromise(executor[$method](params.$args));
    }
    if (typeof executor != 'function')
      throw new Error('executor ' + $exec + ' is not a function');
    return util.toPromise(executor(params.$args));
  }
docs: execute external executor/method
  @this   Evaluation
  @param  {Object} params instruction keywords ($exec, $method, $args)
  @return {Promise|Any} result