null safety

main.dart

class Foo {
  covariant double? _zoo = 3.14;
  
  set zoo(covariant double? o) {_zoo = o;}
  double? get zoo => _zoo;
}

class Bar extends Foo {
  @override double _zoo = 2.3;
  @override set zoo(double o) {_zoo = o;}
  double get zoo => _zoo;
}


void main() {
  Foo x = Bar();
  x.zoo = null;  // Accepted, but will break at runtime.
  print("Never reached.");
}