异步API

　　整合到一起，作为参考：

    //2 5 3 4 1
    //2 5 3 1 4
    var img = new Image();
    img.src = '.';
    img.onerror = function() {
        console.log(1);
    };
    console.log(2);
    Promise.resolve().then(() => console.log(3));
    setTimeout(() => console.log(4), 0);
    Promise.resolve(console.log(5));