anchor.diff:  
   /// Parses setext-style headers.
   class SetextHeaderSyntax extends BlockSyntax {
  +
  +  const SetextHeaderSyntax();
  +
     bool canParse(BlockParser parser) {
       // Note: matches *next* line, not the current one. We're looking for the
       // underlining after this line.
  
positive.code:   bool get canEndBlock => false;
  
    String get listTag;
  
    Node parse(BlockParser parser) {
      final items = <ListItem>[];
  
negative.code: class BlockquoteSyntax extends BlockSyntax {
    RegExp get pattern => _RE_BLOCKQUOTE;
  
    const BlockquoteSyntax();
  
    Node parse(BlockParser parser) {
      final childLines = parseChildLines(parser);
  
  