Remove dart:utf in third_party.

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 'dart:math';
-import 'dart:utf' show codepointsToString;
+import 'package:utf/utf.dart' show codepointsToString;
 import 'dom.dart';
 
 /** A simple tree visitor for the DOM nodes. */
 class TreeVisitor {
   visit(Node node) {
     switch (node.nodeType) {
       case Node.ELEMENT_NODE: return visitElement(node);
       case Node.TEXT_NODE: return visitText(node);
       case Node.COMMENT_NODE: return visitComment(node);
       case Node.DOCUMENT_FRAGMENT_NODE: return visitDocumentFragment(node);
@@ skipping 148 matching lines @@
 bool isVoidElement(String tagName) {
   switch (tagName) {
     case "area": case "base": case "br": case "col": case "command":
     case "embed": case "hr": case "img": case "input": case "keygen":
     case "link": case "meta": case "param": case "source": case "track":
     case "wbr":
       return true;
   }
   return false;
 }