old_contents: import http from 'http'
  import tmpApp from './app'
  import { PORT } from './config'
  
  let app = tmpApp
  const server = http.createServer()
  server.on('request', app)
  server.listen(PORT, () =>
  	console.log('Express server listening on port ' + PORT))
  
  export default server
  
  if (module.hot) {
  	module.hot.accept('./app', () => {
  		let hotApp = null
  		try {
  			hotApp = require('./app')
  		} catch (err) {
  			console.error(err.stack)
  			return
  		}
  		server.removeListener('request', app)
  		app = hotApp
  		server.on('request', app)
  	})
  }
  
new_contents: import http from 'http'
  import tmpApp from './app'
  import { PORT } from './config'
  
  let app = tmpApp
  const server = http.createServer()
  server.on('request', app)
  server.listen(PORT, () =>
  	console.log('Express server listening on port ' + PORT))
  
  export default server
  
  if (module.hot) {
  	module.hot.accept('./app', () => {
  		let hotApp = null
  		try {
  			hotApp = require('./app').default
  		} catch (err) {
  			console.error(err.stack)
  			return
  		}
  		server.removeListener('request', app)
  		app = hotApp
  		server.on('request', app)
  	})
  }
  
subject: fix babel upgrage bug