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

pkg/analyzer/test/generated/non_error_resolver_test.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.non_error_resolver_test;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 3375 matching lines @@
   m(p);
 }
 class B extends A {
   noSuchMethod(v) => '';
 }''');
     computeLibrarySourceErrors(source);
     assertNoErrors(source);
     verify([source]);
   }
 
+  void test_nonAbstractClassInheritsAbstractMemberOne_overridesMethodInObject() {
+    Source source = addSource(r'''
+class A {
+  String toString([String prefix = '']) => '${prefix}Hello';
+}
+class C {}
+class B extends A with C {}''');
+    computeLibrarySourceErrors(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   void test_nonBoolExpression_functionType() {
     Source source = addSource(r'''
 bool makeAssertion() => true;
 f() {
   assert(makeAssertion);
 }''');
     computeLibrarySourceErrors(source);
     assertNoErrors(source);
     verify([source]);
   }
@@ skipping 2112 matching lines @@
     reset();
   }
 
   void _check_wrongNumberOfParametersForOperator1(String name) {
     _check_wrongNumberOfParametersForOperator(name, "a");
   }
 
   CompilationUnit _getResolvedLibraryUnit(Source source) =>
       analysisContext.getResolvedCompilationUnit2(source, source);
 }