I have one idea for a better approach, but it's painful, and I haven't tested it: We
            could postprocess Collect.gwt.xml to add <skip> lines for all the files that should be
            covered by testModule.gwt.xml. Maybe I'll try it someday.

            [*] https://code.google.com/p/google-web-toolkit/wiki/ResourceOracle#When_multiple_PathPrefix_es_have_the_same_path

              <arg>doesnotexist</arg>
              <!-- https://errorprone.info/docs/installation#maven -->
              <arg>-XDcompilePolicy=simple</arg>
              <!-- -Xplugin:ErrorProne is set conditionally by a profile. -->
            </compilerArgs>
            <annotationProcessorPaths>
              <path>
                <groupId>com.google.errorprone</groupId>
                <artifactId>error_prone_core</artifactId>

              <arg>doesnotexist</arg>
              <!-- https://errorprone.info/docs/installation#maven -->
              <arg>-XDcompilePolicy=simple</arg>
              <!-- -Xplugin:ErrorProne is set conditionally by a profile. -->
            </compilerArgs>
            <annotationProcessorPaths>
              <path>
                <groupId>com.google.errorprone</groupId>
                <artifactId>error_prone_core</artifactId>