webpack.config.js

const excludeNodeModulesExcept = function (modules) {
  var pathSep = path.sep;
  if (pathSep == '\\')
    // must be quoted for use in a regexp:
    pathSep = '\\\\';
  var moduleRegExps = modules.map(function (modName) {
    return new RegExp('node_modules' + pathSep + modName);
  });

  return function (modulePath) {
    if (/node_modules/.test(modulePath)) {
      for (var i = 0; i < moduleRegExps.length; i++)
        if (moduleRegExps[i].test(modulePath)) return false;
      return true;
    }
    return false;
  };
};

// Later in the file
...
rules: [
  {
    test: /\.js$/,
    use: [babelLoader],
    exclude: excludeNodeModulesExcept(['debug', '@angular']), // <<<<< THIS PART IS NEEDED
  },
...