We started Flutter in 2014 and made the decision to switch to Dart in ~Jan 2015 iirc.
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.
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.