Custom webpack config alias not working in ionic 2

I am using environment alias in my custom config like this: package.json

 "config": {
    "ionic_bundler": "webpack",
    "ionic_webpack": "./config/webpack-extension.config.js"
  }

webpack-extension.config.js

/*
inspired by:
    https://github.com/driftyco/ionic-app-scripts/pull/683#issuecomment-287401855
*/

let path = require('path')
let useDefaultConfig = require('@ionic/app-scripts/config/webpack.config.js')

module.exports = function () {

useDefaultConfig.resolve.alias = {
    "@app/env": path.resolve(__dirname, '../src/environments/env.' + process.env.IONIC_ENV + '.ts'),
};

return useDefaultConfig;
}

environment.ts

import {

isDEV,
API_HOST,
...etc

} from '@app/env'

But I am getting error:

Typescript Error
Cannot find module '@app/env'.