code: func (h *DeployHandler) serveProgress(w http.ResponseWriter, instanceKey blob.Ref) {
  	h.tplMu.RLock()
  	defer h.tplMu.RUnlock()
  	if err := h.tpl.ExecuteTemplate(w, "withform", &TemplateData{
  		Prefix:      h.prefix,
  		InstanceKey: instanceKey.String(),
  		PiggyGIF:    h.piggyGIF,
  	}); err != nil {
  		h.logger.Printf("Could not serve progress: %v", err)
  	}
  }
docs: // serveProgress serves a page with some javascript code that regularly queries
  // the server about the progress of the requested Google Cloud Engine VM creation.
  // The server replies through serveInstanceState.