The HiddenTaglet has an incorrect method name,
http://hg.openjdk.java.net/jdk10/jdk10/langtools/file/9fa96500eb15/src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/toolkit/taglets/HiddenTaglet.java#l53
should be writer.hiddentTagoutput(holder);