Actually, Dart - with the exception of sound null safety - was created in 2011 and therefore is older than Swift, Kotlin, or Rust and of a similar age as Go.

I'm not sure why you use "Actually" as if you are contradicting something, because you have no idea what I consider as a "newer language". But anyway to take your examples I had to look up their start dates and Swift (2014) was indeed later than Dart, but Go (2009), Rust (2010) and Kotlin (2011) were not.

I personally don't mind that Dart is tied to Flutter.

Fair enough, but the reality is it will effect the uptake of Flutter and Dart, because unlike Objective-C in 2010 there are choices, some of which have vast ecosystems and lots of reasons to use them beyond mobile apps. And it does not help that even on mobile, Flutter has some big issues and seems to be significantly undermanned. Which I'm not sure is simply a reflection of how big a project it is or how much of a mess the project is, probably both.