I guess that at the time you started developing Flutter C#'s hot reload, source generators and NativeAOT compilation didn't yet exist or were just introduced and incomplete.
So before .NET Core 1.0. In 2015 C# still mainly targeted only MS platforms (ignoring Mono) and didn't have mentioned features yet.
I know that Dart started as an alternative to JS, but it seems like JS target is now (unnecessarily) limiting Dart language in a way. I would be nice to be able to use lists of structs or a proper uint64 when needed. As a language it needs to expand in both directions to compete: high level productivity features and low level performance features. It has potential, but it's not there yet.
If Dart would find more use cases besides Flutter, it would make more sense to invest in its ecosystem.
I think you're not wrong about JS target being unnecessarily limiting. The problem is that a 250B/year business is written on top of Dart's JS transpiler (Google Ads) so it seems unlikely to be removed from the language anytime soon
(maybe Dart2Wasm could allow that?).
It's pretty neat that Dart's JS support means you can take your code (e.g Flutter app) to the web, but I think that whole aspect of the ecosystem is underexplored/underdeveloped as of yet.