Show how extends at least `Object` allow to forbid passing dynamic generics

typed_generics.dart

class Generic<T> {
  final T that;
  Generic(this.that);
}

class TypedGeneric<T extends Object> {
  final T that;
  TypedGeneric(this.that);
}

void main() {
  dynamic a;
  Object? b = null;

  print((Generic(a)).that.toString());
  print(Generic<Object?>(a).that.toString());
  print(TypedGeneric(a).that.toString());
  print(TypedGeneric(b).that.toString());
  print(TypedGeneric("ok").that.toString());
}