src/language-html/preprocess.js/addHasHtmComponentClosingTag
function addHasHtmComponentClosingTag(ast, options) {
  return ast.map((node) =>
    node.type !== "element"
      ? node
      : Object.assign(node, {
          hasHtmComponentClosingTag:
            node.endSourceSpan &&
            /^<\s*\/\s*\/\s*>$/.test(
              options.originalText.slice(
                node.endSourceSpan.start.offset,
                node.endSourceSpan.end.offset
              )
            ),
        })
  );
}