code: function getCode(filePath) {
    // take code from cache if available
    const cachedCode = esm.compiler.getCached(filePath);
    if (cachedCode) {
      return cachedCode;
    }
  
    if (!fs.existsSync(filePath)) {
      throw new Error(`File does not exist: ${filePath}`);
    }
  
    // otherwise read code from file and compile it
    const code = fs.readFileSync(filePath, 'utf-8');
    return esm.compiler.compile(filePath, code);
  }
docs: Middleware for serving es modules. Karma doesn't serve any files not specified
  beforehand. This is extremely inconvenient when working with es modules, as many
  files are requested on the fly.