The parameter of synthetic setters should inherit 'covariant'.

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

 // Copyright (c) 2015, 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 file is an "idl" style description of the summary format.  It
  * contains abstract classes which declare the interface for reading data from
  * summaries.  It is parsed and transformed into code that implements the
  * summary format.
  *
@@ skipping 806 matching lines @@
  */
 abstract class LinkedUnit extends base.SummaryClass {
   /**
    * List of slot ids (referring to [UnlinkedExecutable.constCycleSlot])
    * corresponding to const constructors that are part of cycles.
    */
   @Id(2)
   List<int> get constCycles;
 
   /**
-   * List of slot ids (referring to [UnlinkedParam.inheritsCovariantSlot])
-   * corresponding to parameters that inherit `@covariant` behavior from a base
-   * class.
+   * List of slot ids (referring to [UnlinkedParam.inheritsCovariantSlot] or
+   * [UnlinkedVariable.inheritsCovariantSlot]) corresponding to parameters
+   * that inherit `@covariant` behavior from a base class.
    */
   @Id(3)
   List<int> get parametersInheritingCovariant;
 
   /**
    * Information about the resolution of references within the compilation
    * unit.  Each element of [UnlinkedUnit.references] has a corresponding
    * element in this list (at the same index).  If this list has additional
    * elements beyond the number of elements in [UnlinkedUnit.references], those
    * additional elements are references that are only referred to implicitly
@@ skipping 2377 matching lines @@
   /**
    * If this variable is inferable, nonzero slot id identifying which entry in
    * [LinkedLibrary.types] contains the inferred type for this variable.  If
    * there is no matching entry in [LinkedLibrary.types], then no type was
    * inferred for this variable, so its static type is `dynamic`.
    */
   @Id(9)
   int get inferredTypeSlot;
 
   /**
+   * If this is an instance non-final field, a nonzero slot id which is unique
+   * within this compilation unit.  If this id is found in
+   * [LinkedUnit.parametersInheritingCovariant], then the parameter of the
+   * synthetic setter inherits `@covariant` behavior from a base class.
+   *
+   * Otherwise, zero.
+   */
+  @Id(15)
+  int get inheritsCovariantSlot;
+
+  /**
    * The synthetic initializer function of the variable.  Absent if the variable
    * does not have an initializer.
    */
   @Id(13)
   UnlinkedExecutable get initializer;
 
   /**
    * Indicates whether the variable is declared using the `const` keyword.
    */
   @Id(6)
@@ skipping 59 matching lines @@
   @Id(11)
   int get visibleLength;
 
   /**
    * If a local variable, the beginning of the visible range; zero otherwise.
    */
   @informative
   @Id(12)
   int get visibleOffset;
 }