找出监听器移除里的bug

'use strict';

class Dashboard {
  constructor(bus) {
    this.bus = bus;
    this.total = 0;
  }

  render(value) {
    this.total += value;
  }

  start() {
    this.bus.on('data', this.render.bind(this));
  }

  stop() {
    this.bus.off('data', this.render.bind(this));
  }
}