code: func (c *BundlingChecker) partitionByExistence(items []checkerItem) (existing, notExisting []checkerItem) {
  	for _, item := range items {
  		if c.exists(item.item) {
  			existing = append(existing, item)
  		} else {
  			notExisting = append(notExisting, item)
  		}
  	}
  	return existing, notExisting
  }
docs: // partitionByExistence partitions items into two slices: one containing items that
  // are known to already exist on the server, and the other containing the remaining items.