Returns dict associating URL to (size, sha256).

def get_all_sizes_checksums():
  """Returns dict associating URL to (size, sha256)."""
  sizes_checksums = {}
  for path in _checksum_paths().values():
    data = _get_sizes_checksums(path)
    for url, size_checksum in data.items():
      if (url in sizes_checksums and
          sizes_checksums[url] != size_checksum):
        raise AssertionError(
            'URL %s is registered with 2+ distinct size/checksum tuples.' % url)
    sizes_checksums.update(data)
  return sizes_checksums