It's a thing that solved a lot of very real problems that are still halfassed by NPM and Node ecosystem - package manager, build system, tooling around it - it was all a step above the quality of TypeScript/NPM/JS. Saner object semantics, good standard library, good tooling - it just got me to solving the problems I had. DOM wrappers it had were very nice, came with their own wrappers for Observable events (from standard library). They had proper async/await implementation working waay before TS.
Dart feels like a better TypeScript and for me the learning curve was really low.
One issue I had with it was using Dartium to debug the code - Chromium build that ran Dart VM naively to avoid transpiling to JS all the time - it was buggy as hell back then. Supposedly the dev compiler for JS is fast enough now that this isn't an issue - and in the case of Flutter you're running Dart natively so it's irrelevant.
I'm looking forward to doing a new project in Flutter this weekend - I've played with a hello world app when it was in beta and I was very impressed with the dev experience - hopefully it lives up to it in practice.