This problem can creep up in a number of ways, the main issue is that
you do not have the validation-api-1.0.0.GA.jar on your classpath.
I ran into this issue using maven to build my eclipse project. Someone
from the gwt 2.3 forgot to include the validation-api jar as a
dependency in the maven pom
I had the same exact issue.
Check your build path settings. In my case the culprit was 'm2eclipse
maven project import', it was changing a rule in my build path during
import from "src/main/resources exclude: '**/*.java'" to "src/main/
resources exclude: '**'", which was forcing eclipse environmen
