node >= 7.6.0
File System 中异步API 支持 async/await。
每个异步方法，加上后缀Async变成新的方法。
access -> accessAsync
...
使用：
const fs = require('./fs-async-await')
let readFile = async (filename) => {
return await fs.readFileAsync(filename);
}
(async () => {
let content = await readFile(__filename);
console.log(content.toString());
})();