Fix false positives in "Prefer finals over getters" rule.

[pq]

As @zoechi pointed out on #57169, we need to refine our check to ensure we don't flag cases where the field is written to outside a constructor initializer.

In other words, this should produce a lint:

class A {
  var _a;
  var get a => _a; //LINT
  A(this._a);
}

But this should not:

class A {
  var _a;
  var get a => _a; //OK!
  A() {
    _init();
  }

  _init() {
   _a = 42;
  }
}

[xxgreg]

Given library based privacy, I assume you'll also need to check that _a is not set from anywhere in the library, not just from within the class as in the example above.

library foo;
part 'other.dart';
class A {
   var _a;
}

other.dart:

part of foo;
foo(bar) {
  bar._a = 'oi!';
}

Also will there be a way to disable the lint if mirrors are used? (Another tricky case to consider is if generalised tear offs land)

[pq]

@xxgreg 👍

I started implementing this for real today and got far enough along to decide to disable it in the meantime.

Thanks for chiming in!

[pq]

Re-opened #57169. Tracking progress there.