code: func NewCachedBlobStatter(cache distribution.BlobDescriptorService, backend distribution.BlobDescriptorService) distribution.BlobDescriptorService {
  	return &cachedBlobStatter{
  		cache:   cache,
  		backend: backend,
  	}
  }
docs: // NewCachedBlobStatter creates a new statter which prefers a cache and
  // falls back to a backend.