anchor.diff:  
     test_tryFinally_outerEqNullExit_assignUnknown_body() async {
       await trackCode(r'''
  -void f(int a, int b) {
  +void f(int? a, int? b) {
     if (a == null) return;
     try {
       /*nonNullable*/ a;
  
positive.code: 
    test_if_then_else() async {
      await trackCode(r'''
  void f(int x) {
    if (x == null) {
      /*nullable*/ x;
    } else {
  
negative.code: 
    test_assign_toNull() async {
      await trackCode(r'''
  void f(int? x) {
    if (x == null) return;
    /*nonNullable*/ x;
    x = null;
  