c.g.c.base.testModule transitively pulls in .java files from both modules while reading
            the module description of only the prod module. And the prod module doesn't inherit all
            the modules that the test module classes use, so we get errors.

            The good news is that, despite ignoring errors here, GWT does fail if any errors affect

              <compilerArgs combine.children="append">
                <!-- https://errorprone.info/docs/installation#maven -->
                <!-- TODO(cpovirk): Enable NullArgumentForNonNullParameter for
                     prod code. It's disabled automatically for "test code"
                     (which is good: our tests have intentional violations), but
                     Error Prone doesn't know it's building test code unless we