Need to handle skipping mixins in a CustomElement

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

Index: tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
index c97d4f5cc09f7417cadb7be5ab24b6f4ad4a5578..0a0ff4c67eb00b0c0c5a5615eca7cc11313214d2 100644
--- a/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLDocument.darttemplate
@@ -282,7 +282,7 @@ $if DARTIUM
 
         // 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(");
@@ -295,6 +295,10 @@ $if DARTIUM
       }
 
       classMirror = classMirror.superclass;
+      while (classMirror != classMirror.mixin) {
+        // Skip the mixins.
+        classMirror = classMirror.superclass;
+      }
     }
 
     return true;