Consider adding unnecessary_nullable_for_final_variable_declarations

[goderbauer]

Cleaner code, less mental overhead. Enabled in Flutter.

[mit-mit]

Need to fix metadata:

Maturity: experimental
Linter v0.1.118

[mit-mit]

Approved assuming this is only for local variables.

[pq]

Currently it does cover fields and top-level variable declarations in addition to locals.

For fields, would it be sufficient to update it to skip the override case?

/cc @lrhn

[srawlins]

This has a fix.