env(env) {
    if (!env) {
      throw new Error('Env param is required!');
    }
    const factories = Object.getPrototypeOf(this).__envFactories;
    if (factories[env]) {
      return factories[env];
    } else {
      const factory = new WebpackFactory(
        Object.assign({}, this.__webpackConfig),
        Object.assign({}, this.__plugins),
        [].concat(this.__rules)
      );
      factory.env = env;
      Object.getPrototypeOf(this).__envFactories[env] = factory;
      return factory;
    }
  }
return new instance for new env
@param {*} env
@return {Object}