Skip test when custom elements polyfill is in use

Cannot be supported with polyfill because it uses setAttribute.

Author: Steven Orvell

test/unit/legacy-noattributes.html

@@ -117,7 +117,10 @@
         assert.ok(def);
       });
 
-      test('native properties observeable when `_legacyForceObservedAttributes` set', () => {
+      test('native properties observeable when `_legacyForceObservedAttributes` set', function() {
+        if (customElements.polyfillWrapFlushCallback) {
+          this.skip();
+        }
         el = document.createElement('x-native-attrs-force');
         document.body.appendChild(el);
         el.tabIndex = 5;