[analyzer] An external private field is erroneously promoted

[sgrekhov]

The code below produces an error in CFE but no error is reported in the analyzer

// SharedOptions=--enable-experiment=inference-update-2

class C {
  external final int? _x;

  void test() {
    if (_x is int) {
      _x.isOdd; // CFE. Error: Property 'isOdd' cannot be accessed on 'int?' because it is potentially null.
    }
  }
}

main() {
  C().test();
}

Seems that private fields promotion works here but it shouldn't. An external field declaration is effectively an external getter and setter. Nothing guarantees that it will return the same value if read twice, so it must not promote.

See #53419

[stereotype441]

I'll work on this.

[sgrekhov]

@stereotype441 the following test is still failing in CFE and Analyzer

class A {
  void foo() {}
}

class C<T> {
   external final T _x;

  void test() {
    if (_x is A) {
      _x.foo(); // Ok
//       ^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    }
  }
}

main() {
  C c = C();
  if (c._x is A) {
    c._x.foo(); // No expected error
//       ^^^^^
// [analyzer] unspecified
// [cfe] unspecified
  }
  c.test();
}

[stereotype441]

The failure mentioned in #53426 (comment) was addressed by dart-lang/co19#2309.