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?

Start with the linked Plunker reproduction and inspect the Angular Material dialogue behavior on small viewport heights. Compare the current version with the described Material 1 behavior, focusing on vertical margins and content overflow. Done means the dialogue remains within the viewport and its content becomes scrollable when the screen is too short.

Written by the indexing model from the issue text.