The problem is that GWT responds poorly to two .gwt.xml files in the same
    Java package; see https://goo.gl/pRV3Yn for details.

    The summary is that it ignores one file in favor of the other.
    util.concurrent, like nearly all our packages, has two .gwt.xml files: one
    for prod and one for tests. However, unlike our other packages, as of this
    writing it has test supersource but no prod supersource.

              </path>
            </annotationProcessorPaths>
            <!-- Fork:

                 - for JDK8 because we use a javac9 bootclasspath

                 - for JDK9+ because we need args like add-exports
                 -->
            <fork>true</fork>
          </configuration>
        </plugin>
        <plugin>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.2.0</version>