## Context
When upgrading to Next.js 6.1.1-canary.4 and using the `emit-file-loader` included in Next.js, the following error is thrown:
```bash
error in ./src/graveyard/pages/_app.scss
Module build failed (from ../node_modules/next/dist/build/webpack/loaders/emit-file-loader.js):
TypeError: Cannot read property 'context' of undefined
at Object.module.exports (~/project-root/node_modules/next/dist/build/webpack/load
ers/emit-file-loader.js:27:68)
@ ./src/pages/_app.js 35:0-53 156:17-26 157:13-22
@ multi ./pages/_app.js
```
`next.config.js` (shortened):
```js
module.exports = {
webpack: (config, { dev }) => {
config.module.rules.push({
test: /\.scss$/,
use: [
{
loader: 'emit-file-loader',
options: {
name: 'dist/[path][name].[ext].js'
}
},
{
loader: 'babel-loader',
options: {
babelrc: false,
extends: path.resolve(__dirname, './src/.babelrc')
}
},
'styled-jsx-css-loader',
{ loader: 'postcss-loader', options: { sourceMap: dev } },
{
loader: 'sass-loader',
options: {
sourceMap: dev
}
}
]
});
return config;
}
};
```
## Suggested Fix
A quick Google search brought me to a [related issue in `webpack-loader`](https://github.com/webpack-contrib/worker-loader/issues/125). As pointed out in the [Webpack docs](https://webpack.js.org/api/loaders/#this-rootcontext):
> Starting with webpack 4, the formerly `this.options.context` is provided as `this.rootContext`.
This PR makes this change while maintaining backward compatibility.
## System information
Next.js: 6.1.1-canary.4
Node: v9.3.0
Depends on https://github.com/zeit/next-plugins/pull/228
Failing tests are expected as `@zeit/next-css` has to be updated/released first.
This implements rendering of `.css` chunks. Effectively removing the custom document requirement when adding next-css/sass/less/stylus.
* Compile pages to .next/static/<buildid>/pages/<page>
* Fix test
* Export class instead of using exports
* Use constant for static directory
* Add comment about what the middleware does
The prepares for next-server.
I also took this as an opportunity to get all build directory paths from a single location, as they were previously scattered across webpack/babel plugins and loaders.
