npm run dev: Callback must be a function

错误信息如下
```js
Happy[happybabel]: Version: 4.0.1. Threads: 8 (shared pool)
fs.js:124
throw new ERR_INVALID_CALLBACK();
^

TypeError [ERR_INVALID_CALLBACK]: Callback must be a function
at maybeCallback (fs.js:124:9)
at Object.fs.write (fs.js:642:14)
at /Users/www/admin/build/webpack.dev.config.js:12:8
at FSReqWrap.oncomplete (fs.js:136:20)

```

Start by reproducing `npm run dev` and inspect `build/webpack.dev.config.js` at line 12, the location shown in the stack trace. Check the `fs.write` call there and verify the command completes without the `ERR_INVALID_CALLBACK` error.

Written by the indexing model from the issue text.