The parameter of synthetic setters should inherit 'covariant'.

pkg/analyzer/lib/src/summary/link.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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 4317 matching lines @@
  * implied by a field or variable declaration.
  */
 class PropertyAccessorElementForLink_Variable extends Object
     with ReferenceableElementForLink
     implements PropertyAccessorElementForLink {
   @override
   final bool isSetter;
 
   final VariableElementForLink variable;
   FunctionTypeImpl _type;
+  ParameterElementForLink_VariableSetter _parameter;
   List<ParameterElement> _parameters;
 
   PropertyAccessorElementForLink_Variable(this.variable, this.isSetter);
 
   @override
   ConstVariableNode get asConstVariable => variable._constNode;
 
   @override
   DartType get asStaticType => returnType;
 
@@ skipping 23 matching lines @@
       variable.compilationUnit.enclosingElement;
 
   @override
   String get name => isSetter ? '${variable.name}=' : variable.name;
 
   @override
   List<ParameterElement> get parameters {
     if (_parameters == null) {
       _parameters = <ParameterElementForLink_VariableSetter>[];
       if (isSetter) {
-        _parameters.add(new ParameterElementForLink_VariableSetter(this));
+        _parameter = new ParameterElementForLink_VariableSetter(this);
+        _parameters.add(_parameter);
       }
     }
     return _parameters;
   }
 
   @override
   DartType get returnType {
     if (isSetter) {
       return VoidTypeImpl.instance;
     } else {
@@ skipping 22 matching lines @@
     } else {
       return null;
     }
   }
 
   @override
   bool isAccessibleIn(LibraryElement library) =>
       !Identifier.isPrivateName(name) || identical(this.library, library);
 
   @override
-  void link(CompilationUnitElementInBuildUnit compilationUnit) {}
+  void link(CompilationUnitElementInBuildUnit compilationUnit) {
+    if (isSetter && _parameter != null) {
+      if (_parameter.inheritsCovariant) {
+        compilationUnit._storeInheritsCovariant(
+            variable.unlinkedVariable.inheritsCovariantSlot);
+      }
+    }
+  }
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   String toString() => '$enclosingElement.$name';
 }
 
 /**
  * Base class representing an element which can be the target of a reference.
@@ skipping 681 matching lines @@
 }
 
 /**
  * This exception is thrown when [ExprTypeComputer] cannot inference the type.
  */
 class _InferenceFailedError {
   final String message;
 
   _InferenceFailedError(this.message);
 }