docs: def emit(self, signal, message='__nomessagetoken__'):
          """Emit a signal to the frontend.
  
          :param str signal: name of the signal
          :param message: message to send
          :returns: return value from frontend emit function
          :rtype: tornado.concurrent.Future
          """
          # call pre-emit hooks
          if signal == 'log':
              self.log_backend.info(message)
          elif signal == 'warn':
              self.log_backend.warn(message)
          elif signal == 'error':
              self.log_backend.error(message)
  
          return self.emit_to_frontend(signal, message)