Ahh yes, the infamous "somebody"... 🤣
My main reason for commenting is that when reading #3938 I saw that Dart was dismissed by yourself as a niche language that there is little point supporting.
While I think that was true at the time this issue was raised in 2014, it is quite the opposite case now. Anyone using an Android phone has almost certainly used a Dart/Flutter application as Google uses the framework, and thanks to Canonical it's now an option for Linux desktop development too. It's become a solid production language with deployed apps being used the world over.
[There are several PRs for Scintilla lexer, which might be the actual resolve?](https://github.com/ScintillaOrg/lexilla/pulls) I imagine there is some extra legwork to integrate it into Geany as per what the mentioned PRs implemented?
I have no qualms about attempting this myself, but I'm not familiar with how Geany works under the hood, nor am I very attuned with Dart yet (I'm learning it now and was hoping to be able to use Geany). I'd be more tempted to have a look once the Dart PRs for Lexilla are merged. Maybe @yangsfang would like to revisit his PR?