Issue 27367. Fix linking when setter overrides a field.

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 3979 matching lines @@
  * Element representing the parameter of a synthetic setter for a variable
  * resynthesized during linking.
  */
 class ParameterElementForLink_VariableSetter implements ParameterElementImpl {
   @override
   final PropertyAccessorElementForLink_Variable enclosingElement;
 
   ParameterElementForLink_VariableSetter(this.enclosingElement);
 
   @override
+  bool get isCovariant => false;
+
+  @override
   bool get isSynthetic => true;
 
   @override
   String get name => 'x';
 
   @override
   ParameterKind get parameterKind => ParameterKind.REQUIRED;
 
   @override
   DartType get type => enclosingElement.computeVariableType();
@@ skipping 957 matching lines @@
    * there are no type parameters in scope.
    */
   TypeParameterizedElementMixin get _typeParameterContext;
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   String toString() => '$enclosingElement.$name';
 }