fix parsing of semantic template interation w/ shadowdom polyfill

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 887b4dc2ada4263a1a17c3edad9ecc8025e1c9dc..f7cd4fadcf40860883a642e78723f27c63a7f2ce 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -21443,6 +21443,7 @@ class TableSectionElement extends _HTMLElement native "HTMLTableSectionElement"
 // 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
@@ -21555,6 +21556,7 @@ class TemplateElement extends _HTMLElement native "HTMLTemplateElement" {
     _injectStylesheet();
 
     var templateElement = template;
+    templateElement._templateIsDecorated = true;
     var isNative = templateElement is TemplateElement;
     var bootstrapContents = isNative;
     var liftContents = !isNative;
@@ -21567,12 +21569,11 @@ class TemplateElement extends _HTMLElement native "HTMLTemplateElement" {
             '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();