[sebe]

>But doesn't it seem reasonable to use TypeScript for flutter now?

Flutter started as an experiment from chrome team, so their first language was JavaScript, when they move to dart they decided to write the whole framework layer in dart, the engine is in C/C++, so it seems there is no going back to a JS world.

https://flutter.dev/docs/resources/technical-overview#layer-...

With the Dart, they got a VM, GC and other tools and a language team that could grow Dart to suit flutter. They also got stateful hotreload, a Dart idea, which was something that the Flutter team didn't know they wanted.