Update textmate grammar to latest version (#9920)
* Update textmate grammar to latest version
This updates the grammar to the latest version from https://github.com/dart-lang/dart-syntax-highlight and regenerates the goldens.
(I have another upcoming change, but to simplify reviewing, I didn't want to mix it in with previous updates here)
* Remove old reference to new keyword. We don't treat it as a control keyword anymore.
* Update NEXT_RELEASE_NOTES.md
