Fix strong mode error 'Could not infer type parameter'

BUG=
R=kevmoo@google.com

Review URL: https://codereview.chromium.org//2390773002 .
diff --git a/lib/src/treebuilder.dart b/lib/src/treebuilder.dart
index 83e679a..6be687e 100644
--- a/lib/src/treebuilder.dart
+++ b/lib/src/treebuilder.dart
@@ -352,7 +352,7 @@
     // 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;