Fix TreeMap.Node.equals() to allow nodes to be equal to other types of Map.Entry.
While there, eliminate an unnecessary unchecked cast.

Review at http://gwt-code-reviews.appspot.com/1523804

Review by: rchandia@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10527 8db76d5a-ed1c-0410-87a9-c151d255dfc7