Flutter is what rescued Dart from certain death, after Chrome stop caring and AdWords took over from what was left in Dart 1.0, as most well know language designers on the team like Gilad Bracha and Kasper Lund left Google.
That information is about a decade out of date. In practice a bunch of new stuff is using it from the as yet unreleased Fuchsia OS and major products are being rewritten to take advantage of it (Google Earth comes to mind).
You didn't even knew about its use in Nest Hub Smart Displays.
I know how Flutter is implemented, thank you very much.
Most likely is a very thin layer over existing Google Earth C++, unless you have some proof the C++ code was fully rewritten into Dart, beyond a simple tweet.