audit lint semantics in the context of NNBD

[pq]

NNBD will likely have impact on a number of lint rules which will benefit from adjusted semantics and/or deprecation. This issue is a tracking issue for related conversations.

(See #58026 for tracking NNBD preparedness generally.)

General Considerations

Besides nullable types, there are a number of language enhancements that may have effects on existing lints.

• improved flow analysis,
• definite assignment, and
• type promotion

also:

• the late modifier (https://github.com/dart-lang/language/blob/master/accepted/future-releases/nnbd/feature-specification.md#late-fields-and-variables)

Specific Lints

• avoid_init_to_null basic nnbd awareness for avoid_init_to_null linter#1773