fix parsing of semantic template interation w/ shadowdom polyfill

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 7af79d4341494dbd69b33c78adf59d7d3aa38117..357d02557997489cf8c8b9a22e217001b8686d4b 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -22852,6 +22852,7 @@ class TableSectionElement extends _HTMLElement {
 // TODO(jmesserly): move this type to the MDV package? Two issues: we'd lose
 // type annotation on [Element.bindingDelegate], and "mdv" is normally imported
 // with a prefix.
+@Experimental()
 abstract class BindingDelegate {
   /**
    * This syntax method allows for a custom interpretation of the contents of
@@ -22963,6 +22964,7 @@ class TemplateElement extends _HTMLElement {
     _injectStylesheet();
 
     var templateElement = template;
+    templateElement._templateIsDecorated = true;
     var isNative = templateElement is TemplateElement;
     var bootstrapContents = isNative;
     var liftContents = !isNative;
@@ -22975,12 +22977,11 @@ class TemplateElement extends _HTMLElement {
             'attribute templates.');
       }
       templateElement = _extractTemplateFromAttributeTemplate(template);
+      templateElement._templateIsDecorated = true;
       isNative = templateElement is TemplateElement;
       liftRoot = true;
      }
 
-    templateElement._templateIsDecorated = true;
-
     if (!isNative) {
       var doc = _getTemplateContentsOwner(templateElement.document);
       templateElement._templateContent = doc.createDocumentFragment();