const_instance_field

Only static fields can be declared as const.

The analyzer produces this diagnostic when an instance field is marked as being const.

The following code produces this diagnostic because f is an instance field:

class C {
  const int f = 3;
}

If the field needs to be an instance field, then remove the keyword const, or replace it with final:

class C {
  final int f = 3;
}

If the field really should be a const field, then make it a static field:

class C {
  static const int f = 3;
}