constructor(sourceMap: IndexMap) {
    super(sourceMap);
    this._consumers = sourceMap.sections.map((section, index) => {
      const generatedOffset = {
        lines: add0(section.offset.line),
        columns: add0(section.offset.column),
      };
      const consumer = createConsumer(section.map);
      return [generatedOffset, consumer];
    });
  }
A source map consumer that supports "indexed" source maps (that have a
`sections` field and no top-level mappings).