The JS part of the Node internals is usually included in the executable file at linking.
In order to avoid rebuilding Node at each small change of an internal JS, the Node build system allows building an executable without internals and then loading them at each execution:
./configure --node-builtin-modules-path $(pwd)
In this case, the internals will be loaded as an (almost) normal JS script and, when executing it, the Node debugger will return the filename as a special node: URL.

This small patch adds special handling for those URLs so that they can be mapped to the actual files. This allows for convenient setting of breakpoints. It is a very small change that will impact only a few people, but it will still mean a lot to them 😃