Running the code through Babelify before instrumenting it for code
coverage resulted in incorrect statement location metadata. See the
"statementMap" sections of the two files in
https://gist.github.com/robertknight/4ec0f2c3509af61c31ffd574fe16cb74

Since Isparta already supports ES6 internally, it can instrument the
original untranspiled source, the problem can be resolved by switching
the order of the transforms. See
https://github.com/karma-runner/karma-coverage/issues/157#issuecomment-302133802