Skip not classes in mixin superclass constraints.

R=brianwilkerson@google.com

Change-Id: Ic9f3ed3e032839e1dd953be7dd938c9283851bf8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/101466
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
diff --git a/pkg/analyzer/lib/src/dart/element/element.dart b/pkg/analyzer/lib/src/dart/element/element.dart
index b331eb3..20bdd9a 100644
--- a/pkg/analyzer/lib/src/dart/element/element.dart
+++ b/pkg/analyzer/lib/src/dart/element/element.dart
@@ -7344,6 +7344,7 @@
         constraints = onClause.superclassConstraints
             .map((node) => node.type)
             .whereType<InterfaceType>()
+            .where(_isInterfaceTypeInterface)
             .toList();
       }
       if (constraints == null || constraints.isEmpty) {