The parameter of synthetic setters should inherit 'covariant'.

pkg/analyzer/test/src/summary/top_level_inference_test.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ skipping 1512 matching lines @@
 }
 abstract class B {
   void set x(int _);
 }
 class C implements A, B {
   int get x {}
 }
 ''');
   }
 
-  @failingTest
   test_instanceField_inheritsCovariant_fromSetter_field() async {
     var library = await _encodeDecodeLibrary(r'''
 abstract class A {
   num get x;
   void set x(covariant num _);
 }
 class B implements A {
   int x;
 }
 ''');
@@ skipping 652 matching lines @@
 
   Future<LibraryElement> _encodeDecodeLibrary(String text) async {
     String path = _p('/test.dart');
     provider.newFile(path, text);
     UnitElementResult result = await driver.getUnitElement(path);
     return result.element.library;
   }
 
   String _p(String path) => provider.convertPath(path);
 }