fix parsing of semantic template interation w/ shadowdom polyfill

pkg/mdv/test/template_element_test.dart

Index: pkg/mdv/test/template_element_test.dart
diff --git a/pkg/mdv/test/template_element_test.dart b/pkg/mdv/test/template_element_test.dart
index fb640c9127498e9eef81ffad0b01e193998b0442..ff3728c6b489f8a7b9a14abeb255cc76fb09cb17 100644
--- a/pkg/mdv/test/template_element_test.dart
+++ b/pkg/mdv/test/template_element_test.dart
@@ -1118,7 +1118,7 @@ templateElementTests() {
   });
 
   observeTest('NestedIterateTableMixedSemanticNative', () {
-    if (!TemplateElement.supported) return;

[justinfagnani, 2013/07/23 21:05:14]

Does TemplateElement.supported work properly? Does it need to be updated or
removed?

[Jennifer Messerly, 2013/07/23 21:09:32]

I think the issue is that Shadow DOM polyfill creates the appearance that the
HTMLTemplateElement is there, even though HTML parsing doesn't support
<template>. Maybe there should be another property "parsingSupported" or
something. But my understanding is all those properties are going away.

+    if (!parserHasNativeTemplate) return;
 
     var div = createTestHtml(
         '<table><tbody>'