ES6 support with app builder

[venkiiitk]

Hi,

Using these instructions ( https://developer.adobe.com/app-builder/docs/getting_started/first_app/ ) , we created hello world app and trying use ES6 library ( https://www.npmjs.com/package/critical ) but import is failing 

tried by creating .mjs file as ( index.mjs ) 

import {generate} from 'critical'

export default function (params) {
  return {
    statusCode: 200,
    headers: {
      'Content-Type': 'application/json'
    },
    body: {
      LOG_LEVEL: params.LOG_LEVEL,
      message: 'this is a test message'
    }
  }
}

Really blocked with next steps. 
Thanks in advance.