Fix node_validator xss tests for IE and Firefox

tools/dom/src/Html5NodeValidator.dart

Index: tools/dom/src/Html5NodeValidator.dart
diff --git a/tools/dom/src/Html5NodeValidator.dart b/tools/dom/src/Html5NodeValidator.dart
index c935d07ee2cb611987396e5a00abe9783f6884fb..2bbaf415477ab27216753d22b2f52eebad61d6b0 100644
--- a/tools/dom/src/Html5NodeValidator.dart
+++ b/tools/dom/src/Html5NodeValidator.dart
@@ -423,11 +423,11 @@ class _Html5NodeValidator implements NodeValidator {
   }
 
   bool allowsElement(Element element) {
-    return _allowedElements.contains(element._safeTagName);
+    return _allowedElements.contains(Element._safeTagName(element));
   }
 
   bool allowsAttribute(Element element, String attributeName, String value) {
-    var tagName = element._safeTagName;
+    var tagName = Element._safeTagName(element);
     var validator = _attributeValidators['$tagName::$attributeName'];
     if (validator == null) {
       validator = _attributeValidators['*::$attributeName'];