code: func WriteFileMap(ctx context.Context, bs blobserver.StatReceiver, file *Builder, r io.Reader) (blob.Ref, error) {
  	return writeFileMapRolling(ctx, bs, file, r)
  }
docs: // WriteFileMap uploads chunks of r to bs while populating file and
  // finally uploading file's Blob. The returned blobref is of file's
  // JSON blob.