src/language-css/loc.js/calculateLocStart
function calculateLocStart(node, text) {
  if (node.source) {
    if (typeof node.source.sourceIndex === "number") {
      return node.source.sourceIndex; // value-* nodes have this
    }
    return lineColumnToIndex(node.source.start, text) - 1;
  }
  return null;
}