Issue #3474707 by roderik, fago: Remove special handling of "is" attribute...

Issue #3474707 by roderik, fago: Remove special handling of "is" attribute from CustomElementsNormalizer

src/CustomElementNormalizer.php

@@ -56,12 +56,6 @@ class CustomElementNormalizer implements NormalizerInterface {
       unset($result['element']);
     }
 
-    // Support Vue.js "is" attribute for defining element names.
-    if (!empty($result['is'])) {
-      $result['element'] = $result['is'];
-      unset($result['is']);
-    }
-
     // Collect cache metadata. Since the cache metadata object is passed down
     // to slots, custom elements of slots will add their metadata as well.
     $cache_metadata->addCacheableDependency($element);