[Class modifiers] Implement the new class modifiers feature

[itsjustkevin]

Class modifiers

This issue tracks the implementation of the class modifiers feature.

Specs

• Sealed types
• Base-interface-final

Subissues:

• [Class modifiers] Language tests #50726
• [Class modifiers] Implement feature in the CFE #50727
• [Class modifiers] Analyzer implementation #50728
• [Class modifiers] Tests validating as passing (Tracking Via Google Sheet) #50729
• [Class modifiers] Syntax highlighters implementation #50730
• [Class modifiers] DDC implementation #50731
• [Class modifiers] Dart2js implementation #50732
• [Class modifiers] VM implementation #50733
• [Class modifiers] Build system support #50734
• [Class modifiers] Web debugging support #50735
• [Class modifiers] Core library updates #50736
• [Class modifiers] Analysis server / IDE support work #50737
• [Class modifiers] Cider support #50738
• [Class modifiers] Kythe implementation #50739
• [Class modifiers] Angular compiler implementation #50740
• [Class modifiers] IntelliJ implementation #50741
• [Class modifiers] VM service protocol support #50742
• [Class modifiers] VM debugging support #50743
• [Class modifiers] Parser implementation #50744
• [Class modifiers] Linter implementation #58974
• [Class modifiers] package:meta implementation #50746

[munificent]

I'm in the process of moving the base-interface-final proposal over to "accepted" (dart-lang/language#2722). Part of that is also giving it a better name since it now includes "mixin" and "base-interface-final-mixin" is a truly horrible name. We also need a name for the corresponding experiment flag. After some discussion on the PR, we settled on "class modifiers". I went ahead and updated all of the issues I could find from "NCM" to "class modifiers". Does that sound OK?

[eernstg]

Sounds good!