From which version did you upgrade? Probably one that used an older version of GWT, the current one requires the validation JARs (although your widgetset would probably work despite the error messages).
If you have the latest Vaadin plug-in for Eclipse version, try to first downgrade Vaadin version to an old one (pre-6.7), apply the changes and then re-upgrade it to 6.7.8 using Project Properties → Vaadin. That should trigger the loading of the required GWT version and its dependencies, and the validation JARs will be used in widgetset compilation.