[Sealed class] Syntax highlighters implementation

[itsjustkevin]

Cobbling together the list from past issues:

• https://github.com/dart-lang/dart-syntax-highlight/ (@DanTup)
  • Support modifiers + patterns dart-syntax-highlight#38
• https://github.com/highlightjs/highlight.js (@srawlins)
  • issue: [Request] support new Dart language keywords highlightjs/highlight.js#3725
  • PR: enh(dart) Add base, interface, sealed, and when keywords highlightjs/highlight.js#3726
• https://github.com/codemirror/CodeMirror/ (@domesticmouse)
  • codemirror/codemirror5@6a70589
  • codemirror/codemirror5@c17c5f0

[devoncarew]

@itsjustkevin - it would be useful for this issue to list out the various systems to update (or verify that they work as is)

[srawlins]

I added the list we've been using for the last two years.

[srawlins]

A note for tracking: These features are completed "out-of-band": they do not require changes to the Dart SDK or the Flutter repositories, so they can be landed after the Beta 3 deadline. I think it is still good to track them with the Beta 3 milestone for now.

[itsjustkevin]

See #49752

[DanTup]

The textmate grammar changes were done in dart-lang/dart-syntax-highlight#38

[leafpetersen]

Any updates here?

[vsmenon]

@domesticmouse - per checklist above - is there any work here you were planning on? And anything blocking?

[vsmenon]

ping

@domesticmouse @itsjustkevin - do we have anything left here? I don't think this would block a release. Should we move to cleanup?

[vsmenon]

@domesticmouse - closing based on your comment in the other issue.

[domesticmouse]

@vsmenon added https://github.com/codemirror/codemirror5 commits that add keywords