Fix ClassElement.allSupertypes to include interfaces of mixins (issue 29767)

pkg/analyzer/lib/src/dart/element/element.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.dart.element.element;
 
 import 'dart:collection';
 import 'dart:math' show min;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 1136 matching lines @@
       ClassElement currentElement = currentType.element;
       if (!visitedClasses.contains(currentElement)) {
         visitedClasses.add(currentElement);
         if (!identical(currentType, this.type)) {
           supertypes.add(currentType);
         }
         InterfaceType supertype = currentType.superclass;
         if (supertype != null) {
           typesToVisit.add(supertype);
         }
-        for (InterfaceType type in currentElement.interfaces) {
+        for (InterfaceType type in currentType.interfaces) {
           typesToVisit.add(type);
         }
-        for (InterfaceType type in currentElement.mixins) {
-          ClassElement element = type.element;
-          if (!visitedClasses.contains(element)) {
-            supertypes.add(type);
-          }
+        for (InterfaceType type in currentType.mixins) {
+          typesToVisit.add(type);
         }
       }
     }
   }
 
   /**
    * Compute a list of constructors for this class, which is a mixin
    * application.  If specified, [visitedClasses] is a list of the other mixin
    * application classes which have been visited on the way to reaching this
    * one (this is used to detect circularities).
@@ skipping 8764 matching lines @@
 
   @override
   DartObject computeConstantValue() => null;
 
   @override
   void visitChildren(ElementVisitor visitor) {
     super.visitChildren(visitor);
     _initializer?.accept(visitor);
   }
 }