Use of custom TrackByFunction in MatTree does not preserve expansion state when nodes change

#### What is the expected behavior?

Use of trackBy should also ensure that the expansion state does not get lost when nodes are updated.

#### What is the current behavior?

Use of trackBy does not ensure that the expansion state does not get lost when nodes are updated. This is presumably since TreeControl uses an expansionModel which is a SelectionModel where the SelectionModel has no awareness of the TrackByFunction.

#### What are the steps to reproduce?

Relying upon the TreeControl expansionModel:
https://stackblitz.com/edit/angular-5wpr9o-kjnmsn?file=app%2Ftree-flat-overview-example.ts

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?

all

#### Is there anything else we should know?

Trying to store expansion state in the nodes themselves as a workaround doesn't work (unless you transfer the state):
https://stackblitz.com/edit/angular-5wpr9o-ahptjs?file=app%2Ftree-flat-overview-example.ts

Having a custom expansionModel that is aware of the trackBy state works to preserve state, since the TreeControl's expansion-related methods apparently aren't actually used internally by the tree component:
https://stackblitz.com/edit/angular-5wpr9o-pbmk1g?file=app%2Ftree-flat-overview-example.ts

Beginne mit den verknüpften StackBlitz-Reproduktionen, insbesondere app/tree-flat-overview-example.ts, und untersuche, wie MatTree, TreeControl, TrackByFunction und das expansionModel interagieren. Reproduziere den Verlust des Expansionsstatus, wenn Knoten mit einem benutzerdefinierten trackBy aktualisiert werden. Erledigt ist die Aufgabe, wenn der Expansionsstatus nach diesen Aktualisierungen erhalten bleibt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.