The parameter of synthetic setters should inherit 'covariant'.

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 7782 matching lines @@
   ParameterElementImpl_ofImplicitSetter(
       PropertyAccessorElementImpl_ImplicitSetter setter)
       : setter = setter,
         super('_${setter.variable.name}', setter.variable.nameOffset) {
     enclosingElement = setter;
     isSynthetic = true;
     parameterKind = ParameterKind.REQUIRED;
   }
 
   @override
+  bool get inheritsCovariant {
+    PropertyInducingElement variable = setter.variable;
+    if (variable is FieldElementImpl && variable._unlinkedVariable != null) {
+      return enclosingUnit.resynthesizerContext
+          .inheritsCovariant(variable._unlinkedVariable.inheritsCovariantSlot);
+    }
+    return super.inheritsCovariant;
+  }
+
+  @override
   bool get isCovariant {
     if (isExplicitlyCovariant || inheritsCovariant) {
       return true;
     }
     for (ElementAnnotationImpl annotation in setter.variable.metadata) {
       if (annotation.isCovariant) {
         return true;
       }
     }
     return false;
@@ skipping 1228 matching lines @@
 
   @override
   void visitElement(Element element) {
     int offset = element.nameOffset;
     if (offset != -1) {
       map[offset] = element;
     }
     super.visitElement(element);
   }
 }