implement implicit_reopen

[pq]

From the base-interface-final spec:

@reopen lint
...

A metadata annotation @reopen is added to package meta and a lint "require_reopen" implicit_reopen is added to the linter. When the lint is enabled, a lint warning is reported if a class or mixin is not annotated @reopen and it:

Extends or mixes in a class, mixin, or mixin class marked interface or final and is not itself marked interface or final.

[pq]

Updating the proposed name to implicit_reopen as per the conversation in https://dart-review.googlesource.com/c/sdk/+/278888.

/fyi @lrhn @bwilkerson

[pq]

See also: dart-lang/language#2871

[srawlins]

Is this ticket blocked?

[pq]

Yes. I'm scrapping a prototype that duplicates the class modifier induction algorithm in analyzer in favor of a new implementation that will leverage @kallentu's work (underway) to thread that information through the element model (when it's complete).

[srawlins]

Thanks, @kallentu is there a ticket for your work, or a CL?

[kallentu]

@srawlins There isn't. Made a quick one here #51671
CL is still WIP.

[srawlins]

Thanks much!