In lib/loader.js
https://github.com/webpack/css-loader/blob/master/lib/loader.js#L101
this.options may be rewritten by LoaderOptionsPlugin in the following line
https://github.com/webpack/webpack/blob/master/lib/LoaderOptionsPlugin.js#L29
thus this.options.context could be undefined and cause an error.
Should we change it to this.context to fix the bug?