assignment_to_final

'{0}' can't be used as a setter because it's final.

The analyzer produces this diagnostic when it finds an invocation of a setter, but there's no setter because the field with the same name was declared to be final or const.

The following code produces this diagnostic because v is final:

class C {
  final v = 0;
}
​
f(C c) {
  c.v = 1;
}

If you need to be able to set the value of the field, then remove the modifier final from the field:

class C {
  int v = 0;
}
​
f(C c) {
  c.v = 1;
}