More complete generic type support

[samuelsadok]

In 734a064, generic type support was added, but it appears to be incomplete.

As a result, when running something like this:

myController.withExpandToNode(key).withUpdateNode(
          key,
          myController.getNode(key)!.copyWith(expanded: isExpanded),
        );

it throws _TypeError (type 'List<Node<dynamic>>' is not a subtype of type 'List<Node<NodeData>>?' of 'children').

The error happens on this line in the library:

List<Node<T>> updateNode(String key, Node<T> newNode, {Node<T>? parent}) {
// ...
        if (child.isParent) {
          return child.copyWith(  // <==== on this line
            children: updateNode(
              key,
              newNode,
              parent: child,
            ),
          );
        }
// ...
}

I don't have a comprehensive explanation why this happens. From printing runtimeType in various places and stepping through with the debugger, I'm seeing inconsistent output and it almost looks like a bug in the Dart tooling to me. However it seems to have something to do with withExpandToNode(key) returning a compile-time type of TreeViewController<dynamic>, which then results in conflicting types in the subsequent withUpdateNode(). Making the return type explicit fixes my use case.

Dart 3.7
Flutter 3.29