Dartium was cancelled and AdWords team, having just migrated from GWT to AngularDart, saved the Dart team.
Eventually many left the team.
Somewhere at Google, Flutter started and when they decided to replace JavaScript on their original design, they decided to reach out to the Dart team.
So Dart got a new purpose in life, being Flutter's language.
In this process it was rebooted from dynamic language, into a static type one, having JIT and AOT toolchains.
How long Flutter, and by association Dart, remain relevant remains to be seen.
It was actually really fortunate in that for a long time it didn’t have a big community behind it and they just put a lot of very smart language designers on the team where they had ten years to try various approaches and learn from the mistakes of not only themselves but others without a lot of outside noise.
But there’s no other language I would prefer to write applications in. It’s just a really nice mix of ergonomic, expressive and powerful.
Look at the stronghold grip of C/C++ and how long it’s taken Rust to gain a meaningful foothold in those realms for example.
Google wanted to flat replace JS once already, that was the entire origin of Dart. They only pivoted to the cross platform mobile framework as its primary target after it failed to gain traction as a standard