Consistent animation & UX between ExpansionPanel and Tree?

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

feature request

#### What is the expected behavior?

ExpansionPanel has pleasant default animations when the panels expand and shrink and the expand_more/chevron_right indicators rotate. I expected the new tree widget to have a consistent user experience as the other Material components.

#### What is the current behavior?

The subtrees hide and reveal without animation, which is a more jarring UX. Also, the transition of the chevron doesn't animate. (Perhaps it could be made to with additional coding, but ExpansionPanel doesn't require it.)

#### What are the steps to reproduce?

Compare the sample pages between the [Tree Widgets](https://material.angular.io/components/tree/examples) vs. [Expansion Panel Widgets](https://material.angular.io/components/expansion/examples)

#### What is the use-case or motivation for changing an existing behavior?

When I play with hide/show on the expansion panel and compare that to hide/show on a subtree, it just doesn't feel like a consistent user experience. Given (1) how much Material Design emphasizes motion to guide users through spatial relationships, and (2) how there are default animations for ExpansionPanel, Stepper, and Tabs, it really makes Tree appear the oddball out.

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

Angular 6.0.7
Material 6.3.3

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

まず、issue にリンクされている Tree Widgets と Expansion Panel Widgets のサンプルページを比較し、次に Tree widget の展開/折りたたみの動作と、それに対応する ExpansionPanel のアニメーション動作を追跡します。サブツリーの表示状態の変更がアニメーションし、chevron の遷移が期待される Material のインタラクションと一致し、例でも一貫した体験が示されれば完了です。

索引モデルが issue の本文から書いたものです。