code: public String addPostRunDependent(FunctionalTaskItem dependentTaskItem) {
          IndexableTaskItem taskItem = IndexableTaskItem.create(dependentTaskItem);
          this.addPostRunDependent(taskItem);
          return taskItem.key();
      }
docs: Mark the given TaskItem depends on this taskGroup.
  
  @param dependentTaskItem the task item that depends on this task group
  @return key to be used as parameter to taskResult(string) method to retrieve result of
  invocation of given task item.