Dart is a decent language for Flutter, but I’m not sure why you’d want to use it off of the flutter path. Maybe for CLI or other client-side tools? Either way, if you’re not re-using a dart package or using flutter, you absolutely should not pick it as your tool of choice.
The language seems nice syntactically, but I find it really frustrating in practice. It’s good enough for the “happy path” that was envisioned - basic UI hierarchies. It’s clear some team designed the language by first writing examples of what they wanted it to look like when making a UI. I think Swift UI caused similar complexities on Swift. You can confirm that even the contributors think this from reading controversial PRs in GitHub.
The nice thing about flutter, and Dart, is that it’s starting to take a life on outside Google. Google should hand off ownership to a foundation or other organization - especially now that their newest thing is Kotlin Multiplatform. I hear more and more companies writing flutter backends for their UI work, so hopefully it could survive alternative stewardship. I believe some TVs and car navigation systems now use flutter, plus all the google stuff.