NNBD readiness tracking

[pq]

linting non-migrated code

Running linter tests w/ NNBD exposes some issues.

See: dart-archive/linter#1758 for failures in:

• prefer_bool_in_asserts
• void_checks

and an

• Object.noSuchMethod in ResolverVisitor.visitForStatementInScope (fixed in 795949d)

linting migrated code

See: #58032.

resources

• feature spec
• feature roadmap
• language_2 tests

[pq]

/cc @MichaelRFairhurst @bwilkerson