[html5lib] implement querySelector/querySelectorAll

pkg/third_party/html5lib/test/support.dart

 /// Support code for the tests in this directory.
 library support;
 
 import 'dart:io';
 import 'dart:collection';
 import 'package:path/path.dart' as path;
 import 'package:html5lib/src/treebuilder.dart';
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/dom_parsing.dart';
 
@@ skipping 122 matching lines @@
     for (var child in node.nodes) visit(child);
     indent -= 2;
   }
 
   visitDocument(Document node) {
     indent += 1;
     for (var child in node.nodes) visit(child);
     indent -= 1;
   }
 
+  visitDocumentFragment(DocumentFragment node) => visitDocument(node);
+
   visitElement(Element node) {
     _newline();
     _str.write(node);
     if (node.attributes.length > 0) {
       indent += 2;
       var keys = new List.from(node.attributes.keys);
       keys.sort((x, y) => x.compareTo(y));
       for (var key in keys) {
         var v = node.attributes[key];
         if (key is AttributeName) {
           AttributeName attr = key;
           key = "${attr.prefix} ${attr.name}";
         }
         _newline();
         _str.write('$key="$v"');
       }
       indent -= 2;
     }
     visitChildren(node);
   }
 }