[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.

リンクされている StackBlitz の再現コードから始め、trackBy 関数を使用した場合の MatTree と ngFor の動作を比較します。関連する issue #7877 を確認し、ノード配列を置き換えると、追跡対象のノードを保持したまま、レンダリングされたテンプレート内の変更されたプロパティが更新されることを検証します。

索引モデルが issue の本文から書いたものです。