I doubt that complexity of a language is comparable to that of a widget library. After the text input cell and scrolling window it’s basically dumb geometry/drawing code all along.
It's much harder to to implement a language in a proper way than writing a widget library.
But that hardness comes from the level of knowledge one must have in different fields such as compiler theory, cathegory theory, hardware, beside classical computer science and coding.
To write a widget library you just have to know how to code and have some experience.
That being said, the amount of work for implementing a widget library might be higher, but is less qualified work being done.
I dabbled in computer language theory and compilers and it's hard. I have no issues reading source code for widget and GUI libraries.
The Dart team is separate from the Flutter team, though they do interact as Flutter is now the primary use of Dart and as such, influences what is worked on in the Dart team.
