[mat-tree] Nodes are not updated correctly with trackyBy function set

#### Bug, feature request, or proposal:
MatTree does not reflect the changes to the nodes if trackBy function is used. For example, I have several items with trackBy function pointing to their "id" property. If I provide a new array with completely same ids, but with changes to some of the other properties, change detection does not occur.

#### What is the expected behavior?
When used with trackBy function MatTree should trigger change detection on all nodes, just like ngFor with trackBy does.

#### What is the current behavior?
No changes are detected therefore data is not updated in template.

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-z8ubtt

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Latest versions of Angular, Material (check StackBlitz). Any browser.

#### Is there anything else we should know?
Same [issue](https://github.com/angular/material2/issues/7877) was observed last year with MatTable.

Beginne mit der verknüpften StackBlitz-Reproduktion und vergleiche das Verhalten von MatTree mit ngFor, wenn eine trackBy-Funktion verwendet wird. Prüfe das zugehörige Issue #7877 und verifiziere, dass das Ersetzen des Knoten-Arrays geänderte Eigenschaften im gerenderten Template aktualisiert und dabei getrackte Knoten beibehält.

Vom Indexierungsmodell aus dem Issue-Text verfasst.