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 variable or field with the same name was declared to be final.

The following code produces this diagnostic because v is final:

final v = 0;
​
void f() {
  v = 1;
}

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

var v = 0;
​
void f() {
  v = 1;
}