The Source Maps R3 standard states that sourceRoot is “prepended to
the individual entries in the ‘source’ field.” If sources contains file
names, and sourceRoot is intended to refer to a directory but doesn’t end
with a trailing slash, any consumers of the source map are in for a bad
day.

In particular, with the new NODE_V8_COVERAGE feature built-in to node,
using @babel/register results in bogus coverage data, with
…/srcfoo.js coming out where …/src/foo.js was intended.

Thanks to @bcoe for writing up Source maps in Node.js, it was a big
help in tracking down what was going wrong.

References: bcoe/c8#161, #3608