很简单，先把createReadStream给包装一下，然后就可以使用yield来异步获取文件内容了，这个函数我帮你包装好了，拿去就能用。

 var koa=require("koa");
app=koa()
app.use(function*(next){
  this.body = 'hello'
  this.body += yield createReadStream('./file.txt')
  this.body += 'world'
})
app.listen(3000)

//包装一下createReadStream
function createReadStream(){
  var stream=require("fs").createReadStream.apply(null, arguments);
  return function*(end){
    if (end) {
      if (stream.end) stream.end();
      else if (stream.close) stream.close();
      else if (stream.destroy) stream.destroy();
      return;
    }
    return yield read(stream);
  };
  function read(stream) {
    stream.pause();
    return function(done) {
      if (!stream.readable) {
        return done();
      }
      stream.on('data', ondata);
      stream.on('error', onerror);
      stream.on('end', onend);
      stream.resume();
      function ondata(data) {
        stream.pause();
        cleanup();
        done(null, data);
      }
      function onerror(err) {
        cleanup();
        done(err);
      }
      function onend(data) {
        cleanup();
        done(null, data);
      }
      function cleanup() {
        stream.removeListener('data', ondata);
        stream.removeListener('error', onerror);
        stream.removeListener('end', onend);
      }
    }
  }
};