Technically the dart compiler can be called on individual source files, just like in Kotlin. But, the most popular use for dart is probably building mobile applications with Flutter. And that requires the running the build command at the project base folder and never directly at the file level.
That should be doable if a Geany project is open and is based at the right place, see [this](https://wiki.geany.org/howtos/configurebuildmenu) for gory details.
So I guess is there a preference to add these build commands anyway, although they aren't all that useful for the normal development workflow?
There is no requirement, languages are different, tools are different.