Coveralls shows wrong line numbers

In the file reports on coveralls, there's something weird going on:

https://coveralls.io/builds/3469748/source?filename=src%2Flib%2FValidator.js#L137

The line numbers referred to the transpiled output, but the code displayed is the source... It would be good to fix this if possible.

Start by inspecting the Coveralls build linked in the issue and the transpilation or source-map setup associated with src/lib/Validator.js. Compare the reported line numbers with the displayed source and determine whether the coverage report can map generated output back to source; done means Coveralls points to the correct source lines.

Written by the indexing model from the issue text.