don't depend on attachDom existing.

Author: Steven Orvell

lib/mixins/element-mixin.js

@@ -672,7 +672,7 @@ export const ElementMixin = dedupingMixin(base => {
       if (n.attachShadow) {
         if (dom) {
           if (!n.shadowRoot) {
-            if (window.ShadyDOM) {
+            if (window.ShadyDOM && ShadyDOM['attachDom']) {
               ShadyDOM['attachDom'](dom, this, {mode: 'open'});
             } else {
               n.attachShadow({mode: 'open'});