if (source && line != null && column != null) {
      const mappings = this._getFunctionMappings(source);
      if (mappings) {
        const mapping = findEnclosingMapping(mappings, {line, column});
        if (mapping) {
          return mapping.name;
        }
      }
    }
Retrieves a human-readable name for the function enclosing a particular
source location.

When used with the `source-map` package, you'll first use
`SourceMapConsumer#originalPositionFor` to retrieve a source location,
then pass that location to `functionNameFor`.