prefer_final_fields

The private field {0} could be 'final'.

The analyzer produces this diagnostic when a private field is only assigned one time. The field can be initialized in multiple constructors and still be flagged because only one of those constructors can ever run.

The following code produces this diagnostic because the field _f is only assigned one time, in the field's initializer:

class C {
  int _f = 1;

  int get f => _f;
}

Mark the field final:

class C {
  final int _f = 1;

  int get f => _f;
}