Update to new Webkit additions for custom elements

tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index 7c14df1f18800ee2754966106829e019f66331d0..4a95f9a4071f7d572d1f2a3c5fc069b2b633359b 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -380,6 +380,10 @@ $else
       js.JsNative.setProperty(elemProto, 'createdCallback', js.allowInteropCaptureThis(($this) {
         // The created callback has already been called by the very act of passing a JS
         // custom element from JS to Dart.

[Jacob, 2016/04/25 14:54:12]

this previous comment says something conflicting and incorrect.
Add a comment here describing why we have to call this.

+
+        //  Make element's interceptor a CustomElementClass.
+        _blink.Blink_Utils.setInstanceInterceptorCustomUpgrade($this);
+
         if (oldCreatedCallback != null)
           oldCreatedCallback.apply([], thisArg: $this);
       }));