var nanoTime = 0L
    private set

  /** Backlog of tasks to run. Only one task runs at a time. Guarded by [TaskRunner.lock]. */
  private val serialTaskQueue = ArrayDeque<SerialTask>()

  /** The task that's currently executing. Guarded by [TaskRunner.lock]. */
  private var currentTask: SerialTask = TestThreadSerialTask

  /** The coordinator task if it's waiting, and how it will resume. Guarded by [TaskRunner.lock]. */