It's a bit odd to do the time.Sleep in this code because it means that select is often not able to receive on the ch channel:

  select {
      case r := <-ch:
          fmt.Printf("%s was fetched\n", r.url)
          responses = append(responses, r)
          if len(responses) == len(urls) {
              return responses
          }
      default:
          fmt.Printf(".")
          time.Sleep(5e7)
      }

It would be better to use time.Tick

  ticker := time.Tick(50 * time.Millisecond)

  select {
      case r := <-ch:
          fmt.Printf("%s was fetched\n", r.url)
          responses = append(responses, r)
          if len(responses) == len(urls) {
              return responses
          }
      case <-ticker:
          fmt.Printf(".")
      }

Also, there's no need to use a buffered channel. Each goroutine that's handling a URL get can just wait for the send on the channel before terminating.

  ch := make(chan *HttpResponse, len(urls)) // buffered

can just be

  ch := make(chan *HttpResponse)

And another thing that could be done is to not have asyncHttpGets return a slice of *HttpResponse but have it return a channel and then range over it. Then asyncHttpGets could run in its own goroutine and just close the channel when done.