Fix a highlighting bug (dart-lang/source_span#33)

If a span covered a trailing newline and a single additional line, it
went down a code path that resulted in a range error. That code path
has now been fixed.

Closes dart-lang/source_span#32
4 files changed
tree: 83ea4091d040559c399b955ce35d477758c7f46d
  1. pkgs/