Polymer/ShadowDOM — pull request #174 by arv (2013-06-18 15:06:21+00:00)
Title: Fix Firefox 24 issue related to FocusEvent

FF24 does not support `document.createEvent('FocusEvent')` and to create a new
FocusEvent we need to use `new FocusEvent('x')`.

IE on the other hand does not support event constructors so we have to branch
here.

Fixes #159