Fix strong mode error 'Could not infer type parameter'

lib/src/treebuilder.dart

 /// Internals to the tree builders.
 library treebuilder;
 
 import 'dart:collection';
 import 'package:html/dom.dart';
 import 'package:html/parser.dart' show getElementNameTuple;
 import 'package:source_span/source_span.dart';
 import 'constants.dart';
 import 'list_proxy.dart';
 import 'token.dart';
@@ skipping 334 matching lines @@
   }
 
   /// Get the foster parent element, and sibling to insert before
   /// (or null) when inserting a misnested table node
   List<Node> getTableMisnestedNodePosition() {
     // The foster parent element is the one which comes before the most
     // recently opened table element
     // XXX - this is really inelegant
     Node lastTable = null;
     Node fosterParent = null;
-    var insertBefore = null;
+    Node insertBefore = null;
     for (var elm in openElements.reversed) {
       if (elm.localName == "table") {
         lastTable = elm;
         break;
       }
     }
     if (lastTable != null) {
       // XXX - we should really check that this parent is actually a
       // node here
       if (lastTable.parentNode != null) {
@@ skipping 33 matching lines @@
   Document getDocument() => document;
 
   /// Return the final fragment.
   DocumentFragment getFragment() {
     //XXX assert innerHTML
     var fragment = new DocumentFragment();
     openElements[0].reparentChildren(fragment);
     return fragment;
   }
 }