The parameter of synthetic setters should inherit 'covariant'.

pkg/analyzer/lib/src/summary/summarize_ast.dart

 // Copyright (c) 2016, 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 serialization.summarize_ast;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/type.dart' show DartType;
@@ skipping 976 matching lines @@
       b.nameOffset = variable.name.offset;
       b.type = serializeTypeName(variables.type);
       b.documentationComment = serializeDocumentation(documentationComment);
       b.annotations = serializeAnnotations(annotations);
       b.codeRange = serializeCodeRange(variables.parent);
       bool serializeBodyExpr = variable.isConst ||
           variable.isFinal && isField && !isDeclaredStatic ||
           variables.type == null;
       b.initializer =
           serializeInitializerFunction(variable.initializer, serializeBodyExpr);
+      if (isField && !isDeclaredStatic && !variables.isFinal) {
+        b.inheritsCovariantSlot = assignSlot();
+      }
       if (variable.initializer != null &&
           (variables.isFinal || variables.isConst)) {
         b.propagatedTypeSlot = assignSlot();
       }
       bool isSemanticallyStatic = !isField || isDeclaredStatic;
       if (variables.type == null &&
           (variable.initializer != null || !isSemanticallyStatic)) {
         b.inferredTypeSlot = assignSlot();
       }
       b.visibleOffset = scopeNode?.offset;
@@ skipping 416 matching lines @@
 /**
  * A [_TypeParameterScope] is a [_Scope] which defines [_ScopedTypeParameter]s.
  */
 class _TypeParameterScope extends _Scope {
   /**
    * Get the number of [_ScopedTypeParameter]s defined in this
    * [_TypeParameterScope].
    */
   int get length => _definedNames.length;
 }