Fix some type warnings in html library

Fix some type warnings in html library.
Two changes:
* factories for Element are typed to return Element. That way we know they're downcastable to things like CanvasElement.
* switch "final" to an explicit "dom.HTMLElement", so we know it has innerHTML on it.  (from looking at the static tpes, it thought "temp" was just an dom.Element. You're probably familiar with this "feature" of the DOM... ;-) )

Committed: https://code.google.com/p/dart/source/detail?r=1637

[Jennifer Messerly, 2011-11-17 22:17:02]

Sorry about the whitespace noise in the diff--my TextMate is set up to
automatically kill trailing whitespace per our coding conventions.

[Jacob, 2011-11-17 22:20:28]

lgtm