Step towards eliminating com.google.gwt.user.client.Element

This commit changes all methods that used to take a user.Element to
instead take a dom.Element, while using Node.cast() to keep the return
values as user.Element and avoid breaking callers (who might try to
assign to a user.Element variable, for example).

During the next release cycle, it should be possible for users to
update their code the same way.  Then GWT can change its return values
to dom.Element, users can again do the same, and finally GWT can
eliminate user.Element altogether.

Change-Id: Id793420cae4f62c39b13b6bd1b21fd3487dcd81a