When looking up JSNI references, ignore private fields and methods
in superclasses.

Review at http://gwt-code-reviews.appspot.com/279802

Review by: scottb@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7850 8db76d5a-ed1c-0410-87a9-c151d255dfc7