Change the superclass of the translatable version of
junit.framework.AssertionFailedError to match the JVM version,
for consistency when catching java.lang.AssertionError in
testing tools.

Fixes issue 6863.

Review at http://gwt-code-reviews.appspot.com/1565803

Review by: rjrjr@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10694 8db76d5a-ed1c-0410-87a9-c151d255dfc7