Bigger difference than that. Flutter has you writing in high-level components that become native draw calls using Skia. There's an in-progress back-end that outputs to HTML, Canvas, and CSS paint instead of Skia. You aren't going to be using npm much at all.

https://medium.com/flutter-io/hummingbird-building-flutter-f...

If you are using Dart without flutter (eg, overReact), then you're going to use npm a bit more (though dart has better builtin libs which helps a bit on that front). There is a generator that uses typescript definition files to generate dart interopt files.

https://github.com/dart-lang/js_facade_gen