Nested tree does not correctly update

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
Adding a node to the tree and firing a change event should update the tree

#### What is the current behavior?
The tree does not update

#### What are the steps to reproduce?
[StackBlitz link](https://stackblitz.com/edit/angular-kowmq5-pmrkej?file=app/tree-nested-overview-example.ts) line 123

Open the example, and within 10 seconds expand the Applications node.

The following code should add a node, and the node should be displayed:
```ts
database.dataChange.subscribe(data => {
this.nestedDataSource.data = data;
console.log(data); // <-- see screenshot below
});
console.log('Please expand the Applications node within 10 seconds');
setTimeout(() => {
const data = this.nestedDataSource.data;
for (let node of data) {
if (node.filename == "Applications") {
const newnode = new FileNode();
newnode.filename = "ghost";
newnode.type = "gs";
node.children.push(newnode);
console.log("Added component");
}
}
// force change
console.log("Forcing change");
database.dataChange.next(data); // <-- firing event here
}, 10000);
```
After ten seconds the console correctly logs the added component:


But the view does not update:


#### What is the use-case or motivation for changing an existing behavior?
I would expect the tree to mirror the underlying data model.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
It happens on chrome, with the latest version of the library.

#### Is there anything else we should know?
Sorry I could not make the example autoexpand (please see #13027 on why)

Commencez par app/tree-nested-overview-example.ts à la ligne 123 et reproduisez le problème en développant Applications avant la mise à jour différée. Suivez la manière dont nestedDataSource.data et database.dataChange sont gérés après l’ajout du nœud enfant. Le travail est terminé lorsque la vue affiche le nouveau nœud fantôme au déclenchement de l’événement de changement.

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