Sorry for the inconvenience caused.
We are able to reproduce the reported issue " font-awesome icons & ej2-icons but its not working in production build(it shows as [object object])” at our end and we have confirmed it is bug. We have fixed the reported issue and fix for the issue has been included in the patch released on April 23rd 2019. So kindly upgrade to latest version (17.1.42) to resolve the reported issue.
Please get back to us if you have further queries.