Store the covariant keyword flag for parameters and fields into Kernel and resynthesize in analyzer.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, 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 declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 2377 matching lines @@
   /// TODO(paulberry): remove this once there is a corresponding annotation in
   /// the kernel representation.
   bool isSemiSafe = false;
 
   KernelVariableDeclaration(String name, this._functionNestingLevel,
       {Expression initializer,
       DartType type,
       bool isFinal: false,
       bool isConst: false,
       bool isFieldFormal: false,
+      bool isCovariant: false,
       bool isLocalFunction: false})
       : _implicitlyTyped = type == null,
         _isLocalFunction = isLocalFunction,
         super(name,
             initializer: initializer,
             type: type ?? const DynamicType(),
             isFinal: isFinal,
             isConst: isConst,
-            isFieldFormal: isFieldFormal);
+            isFieldFormal: isFieldFormal,
+            isCovariant: isCovariant);
 
   KernelVariableDeclaration.forValue(
       Expression initializer, this._functionNestingLevel)
       : _implicitlyTyped = true,
         _isLocalFunction = false,
         super.forValue(initializer);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
     inferrer.listener.variableDeclarationEnter(this);
@@ skipping 101 matching lines @@
   accept(v) => unsupported("accept", -1, null);
 
   accept1(v, arg) => unsupported("accept1", -1, null);
 
   getStaticType(types) => unsupported("getStaticType", -1, null);
 
   transformChildren(v) => unsupported("transformChildren", -1, null);
 
   visitChildren(v) => unsupported("visitChildren", -1, null);
 }