Strange Dart 9 (1)

main.dart

// Private members are not part of a class's interface, but implementing
// classes need to implement them.
//
// ... except when the class being implemented is in a separate library. See
// the second part: https://gist.github.com/osa1/a6b70f36d1041996c3ff7fa12c7589d8

class A {
  int _a = 1;
  int _b = 2;

  @override
  String toString() => '$_a $_b';
}

// Compile-time error, even though this implements the public interface
// class B implements A {}

// Compile-time error, does not implement setters
// class C implements A {
//   int get _a => 1;
//   int get _b => 2;
// }

// OK
class D implements A {
  int get _a => throw '';
  int get _b => throw '';
  set _a(int a) => throw '';
  set _b(int a) => throw '';
}

// OK
class E implements A {
  noSuchMethod(invocation) => throw '';
}

// The reason why the private fields are needed is because we're allowed to write this:
void wat(A a) {
  print(a._a); // subtypes need to have getter _a
  a._a = 123; // subtypes need to have setter _a
}

void main() {}