dart web toolkit custom open and closed images

What is the recommended way to replace the default open and closed images on a Tree object?

```
import 'package:dart_web_toolkit/ui.dart' as ui;
ui.Tree tree = new ui.Tree();
```

I took a look at the source of Tree.dart and it looks like the abstract class _TreeResource is supposed to be extended and for this purpose, but it is private.
