help(Tree): TreeControl selection model doesn't work for immutable data

Please see this - https://stackoverflow.com/q/65412659/2896495.

I needed a dynamically loading nodes in the angular material tree, so I created my own `GeoDataSource` which internally uses **_@ngrx/component-store_** to handle all the interaction between data source, TreeControl and the tree itself.

Now, the problem is the tree doesn't keep its expansion state because the `abstract class BaseTreeControl` uses `SelectionModel` class and this in turn uses `Set` to keep track of items that are selected. So if node is expanded it just does this:
```
this._selection.add(value); // Set.add(value)
```

When using immutable data (from the component store) those items are always new.

I tried to use `trackBy` optional function:
```
this.treeControl = new FlatTreeControl(
(node) => node.nodeLevel,
(node) => node.expandable, {
trackBy: (node) => node.groupPath // *** groupPath is string (it's an ID) ***
}
);
```

and for this I had to change `FlatTreeControl` to `FlatTreeControl`, otherwise it doesn't compile. But then I got the following error in the **_template_**:

> Type 'FlatTreeControl' is not assignable to type 'TreeControl'

Here's the template:
```

```

Now what? How do I make all this work with immutable data from the store?

Commencez par lire BaseTreeControl et SelectionModel, puis comparez le générique trackBy de FlatTreeControl avec le type TreeControl attendu par le modèle mat-tree. Reproduisez le cas de données immuables décrit dans l’issue et vérifiez que l’état d’expansion persiste lors du remplacement des nœuds, tandis que les types du modèle restent compatibles.

Rédigé par le modèle d'indexation à partir du texte de l'issue.