prefer_final_locals

Local variables should be final.

The analyzer produces this diagnostic when a local variable isn't marked as being final.

The following code produces this diagnostic because the variable s isn't marked as being final:

int f(int i) {
  var s = i + 1;
  return s;
}

Add the modifier final to the variable, removing the var if there is one:

int f(int i) {
  final s = i + 1;
  return s;
}