anchor.diff:    test_constructor_if_then_else() async {
       await trackCode(r'''
   class C {
  -  C(int x) {
  +  C(int? x) {
       if (x == null) {
         /*nullable*/ x;
       } else {
  
positive.code: 
    test_while_eqNull() async {
      await trackCode(r'''
  void f(int x) {
    while (x == null) {
      /*nullable*/ x;
    }
  
negative.code: 
    test_assign_toNull() async {
      await trackCode(r'''
  void f(int? x) {
    if (x == null) return;
    /*nonNullable*/ x;
    x = null;
  