Exclude members from Object when adding members from mixins (issue 23606)

pkg/analyzer/lib/src/generated/resolver.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 engine.resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'constant.dart';
@@ skipping 5751 matching lines @@
             // Include the members from the superclass in the resultMap.
             //
             _recordMapWithClassMembers(map, mixin, false);
             //
             // Add the members from map into result map.
             //
             for (int j = 0; j < map.size; j++) {
               String key = map.getKey(j);
               ExecutableElement value = map.getValue(j);
               if (key != null) {
-                if (resultMap.get(key) == null ||
-                    (resultMap.get(key) != null && !_isAbstract(value))) {
-                  resultMap.put(key, value);
+                ClassElement definingClass = value
+                    .getAncestor((Element element) => element is ClassElement);
+                if (!definingClass.type.isObject) {
+                  ExecutableElement existingValue = resultMap.get(key);
+                  if (existingValue == null ||
+                      (existingValue != null && !_isAbstract(value))) {
+                    resultMap.put(key, value);
+                  }
                 }
               }
             }
           } finally {
             visitedClasses.remove(mixinElement);
           }
         } else {
           // This case happens only when the superclass was previously visited
           // and not in the lookup, meaning this is meant to shorten the compute
           // for recursive cases.
@@ skipping 9442 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }