The global isNaN is not precise at all, and Number.isNaN is an ES6 feature that makes it preferrable to use assert's sameValue for NaN values, as it handles it internally using the comparison.
This function is equivalent to `$ERROR` (which is automatically included
in test environments). Remove the harness file that defines the
function, remove references to the file from test `includes` lists, and
update scripts to instead invoke the `$ERROR` function.
