Implement external and abstract variables

[eernstg]

This is the tracking issue for implementing external variables and abstract instance variables, as specified here. The implementation plan is here.

Subtasks (with check boxes for the ones that aren't issues in this epic):

• Front end
• Analyzer
• Dartfmt
• Formal specification
• Codemirror (Cider, DartPad)
• Dartdoc
• Co19 tests
• IntelliJ
• Grok
• Analysis server
• Update github syntax highlighting (https://github.com/dart-lang/dart-syntax-highlight)
• VS Code
• Other Tools: Validate that build systems, code generators, Angular compiler all work and are updated.

[vsmenon]

@leafpetersen @franklinyow - do we have a particular release we're aiming for yet?

[eernstg]

The feature is controlled by the non-nullable experiment flag, so presumably it's synchronized with null safety.

[leafpetersen]

Yes, this feature is intended to ship with the null safety release, and both the analyzer and the CFE are on board with that timing (@stereotype441 will contribute substantially to the analyzer implementation, which I believe is already in progress).

[vsmenon]

Thanks.

@franklinyow - are we tracking this for null safety beta work?

[franklinyow]

Added this under Beta Epic

[leafpetersen]

Looks like this is close to done. Other than dartpad, any other work we're tracking?

[eernstg]

The language specification work will be done after the stable 2.8 spec is out, and we don't otherwise consider that step to be blocking.

[leafpetersen]

I think this is done.