code: func (c *BundlingChecker) AddItem(item *Item, isolated bool, callback CheckerCallback) {
  	if err := c.bundler.Add(checkerItem{item, isolated, callback}, 0); err != nil {
  		// An error is only returned if the size is too big, but we always use
  		// zero size so no error is possible.
  		panic(err)
  	}
  }
docs: // AddItem adds the given item to the checker for testing, and invokes the provided
  // callback asynchronously. The isolated param indicates whether the given item
  // represents a JSON isolated manifest (as opposed to a regular file).
  // In the case of an error, the callback may never be invoked.