Focus.moveTo, Focus.moveScopeTo only need one argument

<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)**
_Tuesday Oct 27, 2015 at 16:00 GMT_
_Originally opened as https://github.com/flutter/engine/issues/1824_

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We should just get the widget (second argument) from the context (first argument).
