Update to new Webkit additions for custom elements

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 9fac5ebf309ec3345c470933c7c90b09061aa3ca..9307474d0d3881209c75ef003dc9fc0b86a98ad3 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -20146,6 +20146,10 @@ class HtmlDocument extends Document {
       js.JsNative.setProperty(elemProto, 'createdCallback', js.allowInteropCaptureThis(($this) {
         // The created callback has already been called by the very act of passing a JS
         // custom element from JS to Dart.
+
+        //  Make element's interceptor a CustomElementClass.
+        _blink.Blink_Utils.setInstanceInterceptorCustomUpgrade($this);
+
         if (oldCreatedCallback != null)
           oldCreatedCallback.apply([], thisArg: $this);
       }));