@techee commented on this pull request.
@@ -84,6 +84,7 @@ index cd4b23617..af4a73db4 100644
+ &lmCPP, + &lmCss, + &lmD, ++ &lmDart,
Yeah, sorry, I've been blindly following the instructions in HACKING without thinking much for quite some time ;-).
I've just created https://github.com/geany/geany/pull/3991 to better describe how the patch file should be modified when adding a lexer.