code: func (bb *Builder) PopulateParts(size int64, parts []BytesPart) error {
  	return populateParts(bb.m, size, parts)
  }
docs: // PopulateParts sets the "parts" field of the blob with the provided
  // parts.  The sum of the sizes of parts must match the provided size
  // or an error is returned.  Also, each BytesPart may only contain either
  // a BytesPart or a BlobRef, but not both.