Scrollable Blocks should have gravity.

<a href="https://github.com/apwilson"><img src="https://avatars.githubusercontent.com/u/142874?v=3" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [apwilson](https://github.com/apwilson)**
_Friday Aug 21, 2015 at 22:18 GMT_
_Originally opened as https://github.com/flutter/engine/issues/757_

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