toggleable.dart should move to material/

<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)**
_Thursday Oct 22, 2015 at 02:57 GMT_
_Originally opened as https://github.com/flutter/engine/issues/1731_

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