With code splitting, chunkhash of app.js doesn't change when the required js has changed.

app.js

``` js
require.ensure(['./controllers/a'], function(require) {
  var a = require('./controllers/a');
  console.log(a);
});
```

controllers/a.js

``` js
module.exports = 'a';
```

Changing a.js doesn't change the chunkhash of app.js.

https://github.com/fenivana/webpack-chunkhash-issue
