I've done this with Dust (a handlebars competitor) because it had a file loader extension. I've done this with React and preact with webpack because it can be configured to produce a single file for the application with no chunks.

Unless Flutter has a file loader extension or you can inject the needed resources with script tags into an inline HTML field you may be out of luck.

Are you trying to extend the NetSuite UI or publish a page externally for customer/vendor use?