[html5lib] implement querySelector/querySelectorAll

pkg/third_party/html5lib/lib/dom_parsing.dart

 /// This library contains extra APIs that aren't in the DOM, but are useful
 /// when interacting with the parse tree.
 library dom_parsing;
 
 import 'dom.dart';
 import 'src/constants.dart' show rcdataElements;
 
 /// A simple tree visitor for the DOM nodes.
 class TreeVisitor {
   visit(Node node) {
@@ skipping 22 matching lines @@
 
   visitDocumentType(DocumentType node) => visitNodeFallback(node);
 
   visitText(Text node) => visitNodeFallback(node);
 
   // TODO(jmesserly): visit attributes.
   visitElement(Element node) => visitNodeFallback(node);
 
   visitComment(Comment node) => visitNodeFallback(node);
 
-  // Note: visits document by default because DocumentFragment is a Document.
-  visitDocumentFragment(DocumentFragment node) => visitDocument(node);
+  visitDocumentFragment(DocumentFragment node) => visitNodeFallback(node);
 }
 
 /// Converts the DOM tree into an HTML string with code markup suitable for
 /// displaying the HTML's source code with CSS colors for different parts of the
 /// markup. See also [CodeMarkupVisitor].
 String htmlToCodeMarkup(Node node) {
   return (new CodeMarkupVisitor()..visit(node)).toString();
 }
 
 /// Converts the DOM tree into an HTML string with code markup suitable for
@@ skipping 104 matching lines @@
     case "wbr":
       return true;
   }
   return false;
 }
 
 /// Serialize text node according to:
 /// <http://www.whatwg.org/specs/web-apps/current-work/multipage/the-end.html#html-fragment-serialization-algorithm>
 void writeTextNodeAsHtml(StringBuffer str, Text node) {
   // Don't escape text for certain elements, notably <script>.
-  final parent = node.parent;
+  final parent = node.parentNode;
   if (parent is Element) {
     var tag = parent.localName;
     if (rcdataElements.contains(tag) || tag == 'plaintext') {
       str.write(node.data);
       return;
     }
   }
   str.write(htmlSerializeEscape(node.data));
 }