My hack to temporary resolve this issue https://github.com/webpack-contrib/mini-css-extract-plugin/issues/113

concatenate-css.js

const fs = require('fs');
const glob = require('glob');

module.exports = (options = {}) => {
  const collector = {};
  const stuffs = glob.sync(options.inputGlob)
    .map(filepath => fs.readFileSync(filepath, { encoding: 'utf8' }))
    .join('\n')
    // Collect then remove import syntax
    .replace(/@import url\([^;]+\);/g, (match) => {
      collector[match] = true;
      return '';
    })
  ;
  
  // Concatenate import syntax at the top of the file
  const output = Object.keys(collector).join('\n') + stuffs;

  fs.writeFileSync(options.output, output, { encoding: 'utf8' });
};


// Quick hack: If this file is being run directly as a node script, then execute the function immediately
if (__filename.includes(process.argv[1])) {
  module.exports({
    inputGlob: process.argv[2],
    output: process.argv[3],
  });
}