code: func (fc *fakeClock) BlockUntil(n int) {
  	fc.l.Lock()
  	// Fast path: current number of sleepers is what we're looking for
  	if len(fc.sleepers) == n {
  		fc.l.Unlock()
  		return
  	}
  	// Otherwise, set up a new blocker
  	b := &blocker{
  		count: n,
  		ch:    make(chan struct{}),
  	}
  	fc.blockers = append(fc.blockers, b)
  	fc.l.Unlock()
  	<-b.ch
  }
docs: // BlockUntil will block until the fakeClock has the given number of sleepers
  // (callers of Sleep or After)