Polymer Designer: access via in-editor button

@devoncarew 

I've followed the new suit with accessing the button's HTML element in ace.dart for consistency. Let's talk at the tomorrow's meeting about how this should eventually be.

![screen shot 2014-10-27 at 9 54 41 am](https://cloud.githubusercontent.com/assets/5606182/4795680/8cfaf2e8-5dfe-11e4-8706-12fab89adaa0.png)

![screen shot 2014-10-27 at 10 00 04 am](https://cloud.githubusercontent.com/assets/5606182/4795692/9ccdc10a-5dfe-11e4-9985-b0b1597e7c12.png)