</activation>
      <build>
        <plugins>
          <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
              <!-- Under JDK8, we continue to use errorprone's javac9 (even
                   though we don't run Error Prone itself, which no longer
                   supports JDK8!).

                   Why? At some point, presumably after

                    <contains text="@GwtCompatible"/>
                  </fileset>
                </copy>