模仿async，用Generator函数实现

     async function fn(args) {
    // ...
    }

    // 等同于

    function fn(args) {
      return spawn(function* () {
        // ...
      });
    }

实现spawn函数，async函数会返回一个Promise对象

function spawn (genF) {
return new Promise(function (resolve, reject) {
    let gen = genF();
    function execute (nextF) {//执行Generator函数
        let next;
        try {
            next = nextF()
        } catch (e) {
            reject(e);
        }
        if (next.done) {
            return resolve(next.value);
        }
        Promise.resolve(next.value).then(
            function (v) {
                execute(function () {
                    return gen.next(v)
                })
            },
            function (e) {
                execute(function () {
                    return gen.throw(e)
                })
            })
    }
    execute(function () {
        return gen.next(undefined);
    })
});

}