checkJs too strict about module.exports assignment

I was trying to run with `// @ts-check` on a webpack config file. Unfortunately, it choked on the following simplified repro:

```js
// @ts-check

module.exports = {
    a: 100,
    b: 200,
}

module.exports.devtool = true;
```

I get

```
severity: 'Error'
message: 'An export assignment cannot be used in a module with other exported elements.'
at: '3,1'
source: 'js'
```