The widgets are written in Dart, not C++. If you need a new widget, you would write it in Dart, which is the same as the application language. This contrasts with React Native, where new widgets/views would be written natively and wrapped in React Native.
I’m going to be a little bit pedantic, but that’s untrue. On React (DOM, Native…) you create your widgets with React, and compose them as components.
If you need to go deeper (for some specific perf issue, usually for animation stuff) then you go native. But it’s a second class approach — well, it has first class support and is not as “second class” as it is for example in Titanium or “completely out of scope” in Cordova.
