public ImmutableList<String> getUnfinishedLeafTasks() {
      return unfinishedLeafTasks;
    }
Gets a list of the unfinished "leaf" tasks in the order in which those tasks were
encountered.

@return a list of unfinished "leaf" tasks
jib-core/src/main/java/com/google/cloud/tools/jib/event/progress/ProgressEventHandler.java