Dart is pretty much 'done' as a language though. They are adding code generation and other nice things, but it's stable. Flutter on the other hand needs to be kept up to date as Android and iOS change, so it's very vulnerable.

What is an example of a mainstream language being "done" before?

Most mainstream languages are "done" in the way that matters here. Not "done" in the sense of "not receiving new features anymore", but in the sense that you could switch their implementations to maintenance mode today and they'd remain useful for a long time. Easily visible with languages where people do indeed keep using old versions for whatever reason.

Common Lisp