code: static SourceFile extractSourceMap(
        SourceFile jsFile, String sourceMapURL, boolean parseInlineSourceMaps) {
      // Javascript version of the compiler can only deal with inline sources.
      if (sourceMapURL.startsWith(BASE64_URL_PREFIX)) {
        byte[] data =
            BaseEncoding.base64().decode(sourceMapURL.substring(BASE64_URL_PREFIX.length()));
        String source = new String(data, StandardCharsets.UTF_8);
        return SourceFile.fromCode(jsFile.getName() + ".inline.map", source);
      }
      return null;
    }
docs: For a given //# sourceMappingUrl, this locates the appropriate sourcemap on disk. This is use
  for sourcemap merging (--apply_input_source_maps) and for error resolution.