No, I'm not setting it to null.  Here is an experiment that shows what I think you are asking for and what it does.  Please let me know if I'm misunderstanding this.  But if I do the following as I think you are suggesting:

````dart
class Item {
  String text = '';
  bool done = false;

  Item([this.text, this.done]);
}
````

And then do the following:

````dart
void main() {
  var newItem = new Item('new name');
  print(newItem.text);
  print(newItem.done);
}
````

It will print:
new name
null

So initializing the default values in this way is not past to instances of the Item class.  So... I'm sure I'm brain dumping something here, but what?  :)