Dialogue - overflow scrolling on small screens and margins

#### Bug, feature request, or proposal:
The current dialogue does not seem to support sizing and scrolling on smaller screen sizes / small devices. This was possible on the material 1 dialogue. If the vertical height of the screen was incredibly small scrollbars would appear see this screenshot:


With version 2 - the dialogue is pushed off the screen and no scrollbars ever appear in the middle. It does not maintain any margins on the top or bottom.


Maybe there is this a means this can be achieved but the examples are quite primitive.

#### What is the expected behavior?
When screen is decreased in size scrollbars should appear for the content.

#### What is the current behavior?
The current dialogue is pushed off the screen with no margin at the top or bottom. The content remains the same size

#### What are the steps to reproduce?

Providing a Plunker (or similar) is the *best* way to get the team to see your issue.
https://plnkr.co/edit/DrI42Su0v6OntJkjUNOI?p=preview

#### What is the use-case or motivation for changing an existing behavior?
The current modal is only useful for very simple use cases.

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

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

リンクされた Plunker の再現から始め、viewport の高さが小さい場合の Angular Material のダイアログの動作を確認します。現在のバージョンを、説明されている Material 1 の動作と比較し、垂直方向のマージンとコンテンツのオーバーフローに注目します。画面の高さが足りない場合でもダイアログが viewport 内に収まり、コンテンツがスクロール可能になれば完了です。

索引モデルが issue の本文から書いたものです。