Field promotion doesn't work properly for fields declared in mixins

[stereotype441]

The following code is accepted by the analyzer but rejected by the VM:

mixin M {
  final int? _field = 0;
}

class C extends Object with M {
  void test() {
    if (_field != null) {
      _field.isEven;
    }
  }
}

main() {
  C().test();
}

The error message from the VM states:

../../tmp/proj/test.dart:8:14: Error: Property 'isEven' cannot be accessed on 'int?' because it is potentially null.
Try accessing using ?. instead.
      _field.isEven;
             ^^^^^^

The analyzer is correct--the field _field should be promotable.

I'm working on a fix.

[stereotype441]

A similar problem happens for abstract fields, e.g.:

mixin M {
  abstract final int? _field;
}

abstract class C = Object with M;

test(C c) {
  if (c._field != null) {
      c._field.isEven;
  }
}

main() {}

I'm preparing a CL that addresses both issues.