The issue below confirms that the gwt-dev jar uses a modified version
of flute that allows underscores in @def names (and other places
too?), so letting the classloader find the incubator's version of
flute the prevents the OPACITY_DURATION definition from being parsed
properly. I guess this could have been a problem with pre-2.4 too,
and maybe I've just been lucky enough to never have used any widgets
that have an underscore in thier CSS @defs.
http://code.google.com/p/google-web-toolkit/issues/detail?id=3946
Anyways, this thread is a little old so maybe this post is too late,
but hopefully it can help someone else in the future.
Thanks for pointing this out. I had the exact same problem.