Make the package strong-mode clean.

This caught a bug in which we were calling _FileSpan._start and
_FileSpan._end on a span we couldn't guarantee to be a _FileSpan.

R=sigmund@google.com

Review URL: https://codereview.chromium.org//1328583002 .
3 files changed
tree: 98b7356a49f44ae32b971c1f8c123d320e630707
  1. pkgs/