join_with_return_assignment false positive on complex statements

[srawlins]

Take the following code:

class A {
  int f;
  int g;
}

class B {
  final A a = A();
}

int main() {
  final B b1 = B();
  final B b2 = B();

  b1.a
    ..f = 1
    ..g = 1;
  b2.a.f = 2; // Error here
  return b1.a.f;
}

The join_return_with_assignment rule reports at the error comment "Join return statement with assignment when possible," because it does not see that b1.a is not equal to b2.a.

(Same cause as #57855)