Request: `unused_field` for public properties, methods of private classed

[rrousselGit]

The unused_field lint and its variants are very useful. But they have a flaw: They work only for private members.

Consider:

class _A {
  int unused;
}

Where _A is used but _A.unused is not, then in that situation we do not know that unused is not used. Since _A is private, it should be feasible to know if the field unused is used or not.

A naive first implementation could consider that a public member of a private class is unused if:

• it's not used in the file + its parts
• it is not inherited from a super-class (not annotated with @override)
• the class it belongs to is not implemented/extended
• There are no public instances of the private class available