Use c.g.g.core.shared.GWT in shared code.
Fixes NoClassDefFoundErrors when using shared code without gwt-user.jar.

Contributed by jat
Fixes issue 7527

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11255 8db76d5a-ed1c-0410-87a9-c151d255dfc7