code: func (f *FakeClock) After(d time.Duration) <-chan time.Time {
  	f.lock.Lock()
  	defer f.lock.Unlock()
  	stopTime := f.time.Add(d)
  	ch := make(chan time.Time, 1) // Don't block!
  	f.waiters = append(f.waiters, fakeClockWaiter{
  		targetTime: stopTime,
  		destChan:   ch,
  	})
  	return ch
  }
docs: // Fake version of time.After(d).