anchor.diff:  class UnorderedListSyntax extends ListSyntax {
     RegExp get pattern => _RE_UL;
     String get listTag => 'ul';
  +
  +  const UnorderedListSyntax();
   }
   
   /// Parses ordered lists.
   class OrderedListSyntax extends ListSyntax {
     RegExp get pattern => _RE_OL;
     String get listTag => 'ol';
  +
  +  const OrderedListSyntax();
   }
   
   /// Parses paragraphs of regular text.
   class ParagraphSyntax extends BlockSyntax {
     bool get canEndBlock => false;
  +
  +  const ParagraphSyntax();
   
     bool canParse(BlockParser parser) => true;
   
  
positive.code: /// Parses atx-style headers: `## Header ##`.
  class HeaderSyntax extends BlockSyntax {
    RegExp get pattern => _RE_HEADER;
  
    Node parse(BlockParser parser) {
      final match = pattern.firstMatch(parser.current);
  
negative.code: class EmptyBlockSyntax extends BlockSyntax {
    RegExp get pattern => _RE_EMPTY;
  
    const EmptyBlockSyntax();
  
    Node parse(BlockParser parser) {
      parser.advance();
  
  