Fix Dart syntax highlighting issue (#8347)

Release Notes:
* Improved Dart syntax highlighting
([#8151](https://github.com/zed-industries/zed/pull/8347#8151))

Before:
<img width="1246" alt="Before"
src="https://github.com/zed-industries/zed/assets/25414681/d9143fe8-c474-40bb-afbd-56fef16edab3">

After:
<img width="1249" alt="After"
src="https://github.com/zed-industries/zed/assets/25414681/4b103c35-fb24-4693-8b9e-3dd494036c9f">


(Shameless plug, since I build the theme)
Theme: The Dark Side