add Element.xtag to public interface

Add Element.xtag to public interface

This lets us refer to it easily, instead of via (element as Dynamic).xtag, which in turn allows users to think about it as the Element being the source of truth, and storing a reference to the Element instead of the WebComponent object.

I opened up the doc comment in Firefox to double check that it renders correctly in API doc.

Committed: https://code.google.com/p/dart/source/detail?r=12917

[vsm, 2012-09-26 02:54:42]

lgtm