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?

Inizia confrontando le pagine di esempio Tree Widgets e Expansion Panel Widgets collegate nell’issue, quindi analizza il comportamento di espansione/compressione del Tree widget e il comportamento di animazione corrispondente di ExpansionPanel. Il lavoro è completato quando i cambiamenti di visibilità dei sottoalberi sono animati e la transizione del chevron corrisponde all’interazione Material prevista, con esempi che mostrano un’esperienza coerente.

Scritto dal modello di indicizzazione a partire dal testo della issue.