Do you use Dart for your whole website? How about page load times for the homepage? Or do you use Dart for pages / things that won't appear until the user knows what everything is about and is interested already, and won't mind ... 200 extra ms to download the Dart runtime?

Or is it an internal in-company application?

Do you happen to know how many kb JS code gets downloaded to a browser that doesn't support Dart natively?

(Unless this is too classified :-) )

Yes weird things happens sometimes with JS... a little bit more often with JS than with TS I think because TS is more restrictive.