code: function mapSources (sourceMap, mapCallback) {
    let chain = Promise.resolve()
    function pushChain (path, index, arr) {
      chain = chain
        .then(() => mapCallback(path))
        .then(replacement => {
          arr[index] = replacement || path
        })
    }
    if (sourceMap.sources) {
      sourceMap.sources.forEach(pushChain)
    }
    if (sourceMap.sections) {
      sourceMap.sections.forEach(section => {
        if (section.map && section.map.sources) {
          section.map.sources.forEach(pushChain)
        }
      })
    }
    return chain.then(() => sourceMap)
  }
docs: Replaces (maps over) the source paths from the source map.
  
  @param {object} sourceMap The source map contents
  @param {function} mapCallback Map the values of the source paths