old_contents: var koa = require('koa');
  var app = koa();
  
  app.use(function *() {
      console.log('some is coming');
      this.body = 'hello world';
  });
  
  app.listen(3000, function() {
      console.log('server is listening');
  });
  
new_contents: var koa = require('koa');
  var app = koa();
  
  app.use(function *() {
      console.log('some is coming');
      this.body = 'hello world';
  });
  
  app.listen(8080, function() {
      console.log('server is listening');
  });
  
subject: 修改端口为8080