Because the tree-sitter-dart crate is associated to a discontinued fork of this grammar, we weren't able to use the latest version of this grammar in Mergiraf.
To address this, I have made a fork of this grammar in an organization designed to facilitate collective maintenance of tree-sitter parsers. It's on Codeberg (which works pretty much like Github, you shouldn't be too lost): https://codeberg.org/grammar-orchard/tree-sitter-dart-orchard
The corresponding crate is at https://crates.io/crates/tree-sitter-dart-orchard
If you have any interest in joining, I'd be happy to give you accesses.
Thank you for your work on this grammar!