[Sealed class] Implement the Sealed Class feature

[itsjustkevin]

Feature spec

• [Sealed class] Language tests
• [Sealed class] Implement feature in the CFE
• [Sealed class] Analyzer implementation
• [Sealed class] Tests validating as passing(Tracking Via Google Sheet)
• [Sealed class] Syntax highlighters implementation
• [Sealed class] DDC implementation
• [Sealed class] Dart2js implementation
• [Sealed class] VM implementation
• [Sealed class] Build system support
• [Sealed class] Web debugging support
• [Sealed class] Core library updates
• [Sealed class] Analysis server / IDE support work
• [Sealed class] Cider support
• [Sealed class] Kythe implementation
• [Sealed class] Angular compiler implementation
• [Sealed class] IntelliJ implementation
• [Sealed class] VM service protocol support
• [Sealed class] VM debugging support
• [Sealed class] Parser implementation
• [Sealed class] Language feature docs

[vsmenon]

@leafpetersen - anything you see here as blocking?

[leafpetersen]

Nothing here is release blocking. All of the remaining items should be ticked off before we close this, and preferably before stable.

[mit-mit]

cc @itsjustkevin, I think this can be closed?