private String sourceLevel = USER_LEVEL;
    private boolean sourceLevelSet = false;

    public void setSourceLevel(String sourceLevel) {
        if (sourceLevelSet) {
            throw new IllegalStateException("Cannot reset sourceLevel attribute; it is already set to: " + sourceLevel);
        } else if (!(USER_LEVEL.equals(sourceLevel) || PROJECT_LEVEL.equals(sourceLevel)  || GLOBAL_LEVEL.equals(sourceLevel))) {

              <logLevel>${gwt.logLevel}</logLevel>
              <generateJsInteropExports>true</generateJsInteropExports>
              <validateOnly>true</validateOnly>
              <sourceLevel>1.8</sourceLevel>
              <!--
                To avoid "java.io.IOException: User limit of inotify watches reached" under JDK10.
                Presumably we don't need inotify, since we don't intend to change files during the