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

pkg/kernel/lib/clone.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 kernel.clone;
 
 import 'ast.dart';
 import 'type_algebra.dart';
 
 /// Visitor that return a clone of a tree, maintaining references to cloned
 /// objects.
@@ skipping 342 matching lines @@
   }
 
   visitYieldStatement(YieldStatement node) {
     return new YieldStatement(clone(node.expression));
   }
 
   visitVariableDeclaration(VariableDeclaration node) {
     return variables[node] = new VariableDeclaration(node.name,
         initializer: cloneOptional(node.initializer),
         type: visitType(node.type),
+        isCovariant: node.isCovariant,
         isFinal: node.isFinal,
         isConst: node.isConst,
         isFieldFormal: node.isFieldFormal);
   }
 
   visitFunctionDeclaration(FunctionDeclaration node) {
     var newVariable = clone(node.variable);
     return new FunctionDeclaration(newVariable, clone(node.function));
   }
 
@@ skipping 16 matching lines @@
         isConst: node.isConst,
         transformerFlags: node.transformerFlags,
         fileUri: node.fileUri)
       ..fileEndOffset = node.fileEndOffset;
   }
 
   visitField(Field node) {
     return new Field(node.name,
         type: visitType(node.type),
         initializer: cloneOptional(node.initializer),
+        isCovariant: node.isCovariant,
         isFinal: node.isFinal,
         isConst: node.isConst,
         isStatic: node.isStatic,
         hasImplicitGetter: node.hasImplicitGetter,
         hasImplicitSetter: node.hasImplicitSetter,
         transformerFlags: node.transformerFlags,
         fileUri: node.fileUri)
       ..fileEndOffset = node.fileEndOffset;
   }
 
@@ skipping 24 matching lines @@
   visitArguments(Arguments node) {
     return new Arguments(node.positional.map(clone).toList(),
         types: node.types.map(visitType).toList(),
         named: node.named.map(clone).toList());
   }
 
   visitNamedExpression(NamedExpression node) {
     return new NamedExpression(node.name, clone(node.value));
   }
 }