Scaffold should automatically insert placeholders for bottom sheets

<a href="https://github.com/Hixie"><img src="https://avatars.githubusercontent.com/u/551196?v=3" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [Hixie](https://github.com/Hixie)**
_Friday Nov 06, 2015 at 00:03 GMT_
_Originally opened as https://github.com/flutter/engine/issues/1978_

---
