The parameter of synthetic setters should inherit 'covariant'.

pkg/analyzer/test/generated/strong_mode_driver_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 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_mode_test.dart';
 
 main() {
   defineReflectiveSuite(() {
@@ skipping 40 matching lines @@
   }
 
   @failingTest
   @override
   test_genericMethod_functionInvocation_inferred() {
     return super.test_genericMethod_functionInvocation_inferred();
   }
 
   @failingTest
   @override
-  test_genericMethod_override_covariant_field() async {
-    return super.test_genericMethod_override_covariant_field();
-  }
-
-  @failingTest
-  @override
   test_genericMethod_tearoff() {
     return super.test_genericMethod_tearoff();
   }
 }
 
 @reflectiveTest
 class StrongModeTypePropagationTest_Driver
     extends StrongModeTypePropagationTest {
   @override
   bool get enableNewAnalysisDriver => true;
 }