I think the main problem for Dart was that Typescript is simply the better approach for fixing Javascript (even though TS has its own share of problems with its 'kitchen sink type system').
> even though TS has its own share of problems with its 'kitchen sink type system'
Eh, people critique the kitchen sink type system but at the end of the day that system is why TypeScript took over, not a weird add-on you have to put up with to get it. It's what makes it possible to assign comprehensive types to ~every JS library, which is what allows TypeScript to gradually eat the whole JS ecosystem.
Any other approach stops short of the goal of being JS+Types, which means you end up with a different language with different semantics, which is where Dart failed.
Agreed for enabling gradually adding types to large JS code bases, but at the same time it's also used for very questionable 'template metaprogramming hacks' (which then result in pages-long and completely undecipherable error messages). It's a very similar problem to 'abusing' the C++ template system.
