GWT Error: “No source code is available for type x.y.z; did you forget to inherit a required module?”

“No source code is available for type x.y.z; did you forget to inherit a required module?” Check to make sure x.y.z is actually in the “client” package. GWT doesn’t allow you to use java source files from other packages when the code involved is client code- quite logical when you think about it, but the error isn’t that helpful in explaining this!