Need to handle skipping mixins in a CustomElement

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
 $!MEMBERS
@@ skipping 264 matching lines @@
         createdParametersValid = true;                // Assume no parameters.
         if (methodMirror.parameters.length != 0) {
           // If any parameters each one must be optional.
           methodMirror.parameters.forEach((parameter) {
             createdParametersValid = createdParametersValid && parameter.isOptional;
           });
         }
 
         // Get the created constructor source and look at the initializer;
         // Must call super.created() if not its as an error.
-        var createdSource = methodMirror.source.replaceAll('\n', ' ');
+        var createdSource = methodMirror.source?.replaceAll('\n', ' ');
         RegExp regExp = new RegExp(r":(.*?)(;|}|\n)");
         var match = regExp.firstMatch(createdSource);
         superCreatedCalled = match.input.substring(match.start,match.end).contains("super.created(");
       }
 
       if (!superCreatedCalled) {
         throw new DomException.jsInterop('created constructor initializer must call super.created()');
       } else if (!createdParametersValid) {
         throw new DomException.jsInterop('created constructor must have no parameters');
       }
 
       classMirror = classMirror.superclass;
+      while (classMirror != classMirror.mixin) {
+        // Skip the mixins.
+        classMirror = classMirror.superclass;
+      }
     }
 
     return true;
   }
 $endif
 
   @Experimental()
   /**
    * Register a custom subclass of Element to be instantiatable by the DOM.
    *
@@ skipping 176 matching lines @@
   /// parameter must be provided.
   @Experimental()
   ElementUpgrader createElementUpgrader(Type type, {String extendsTag}) {
 $if DART2JS
     return new _JSElementUpgrader(this, type, extendsTag);
 $else
     return new _VMElementUpgrader(this, type, extendsTag);
 $endif
   }
 }