code: function insert(outMap, inMap, length, index) {
    // Move existing mappings
    if (length) {
      outMap._mappings.unsortedForEach(mapping => {
        if (mapping.generatedLine > index) mapping.generatedLine += length;
      });
    }
  
    // Add new mappings
    if (inMap) {
      const inConsumer = new SourceMapConsumer(inMap.toJSON());
  
      inConsumer.eachMapping(mapping => {
        outMap.addMapping({
          source: mapping.source,
          original: mapping.source == null
            ? null
            : {
                line: mapping.originalLine,
                column: mapping.originalColumn
              },
          generated: {
            line: index + mapping.generatedLine - 1,
            column: mapping.generatedColumn
          }
        });
      });
  
      inConsumer.sources.forEach((source, idx) => {
        outMap.setSourceContent(source, inConsumer.sourcesContent[idx]);
      });
    }
  }
docs: Insert line in 'outMap' with 'inMap' at line 'index'
  @param {SourceMapGenerator} outMap
  @param {SourceMapGenerator} [inMap]
  @param {Number} length
  @param {Number} index