Use attachShadow({shadyUpgradeFragment})

Author: Steven Orvell

lib/mixins/element-mixin.js

@@ -672,12 +672,8 @@ export const ElementMixin = dedupingMixin(base => {
       if (n.attachShadow) {
         if (dom) {
           if (!n.shadowRoot) {
-            if (window.ShadyDOM && ShadyDOM['attachDom']) {
-              ShadyDOM['attachDom'](dom, this, {mode: 'open'});
-            } else {
-              n.attachShadow({mode: 'open'});
-              n.shadowRoot.appendChild(dom);
-            }
+            n.attachShadow({mode: 'open', shadyUpgradeFragment: dom});
+            n.shadowRoot.appendChild(dom);
           }
           if (syncInitialRender && window.ShadyDOM) {
             ShadyDOM.flushInitial(n.shadowRoot);