main.dart

class Number {
  int number;
  Number(this.number);
}

void foo(Number n){
  n = Number(2); // Here you erase your reference and refer to a new Number
}

void bar(Number n){
  n.number = 3; // Here you mutate the "number" of the Number that was passed to you
}

void main(){
  var n = Number(1);
  print(n.number);
  foo(n);
  print(n.number);
  bar(n);
  print(n.number);
}