val existingTask = activeTask
      if (existingTask is AwaitIdleTask) {
        return existingTask.latch
      }
      for (futureTask in futureTasks) {
        if (futureTask is AwaitIdleTask) {
          return futureTask.latch
        }
      }

      // Don't delegate to schedule() because that enforces shutdown rules.
      val newTask = AwaitIdleTask()