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

pkg/kernel/lib/text/ast_to_text.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.ast_to_text;
 
 import '../ast.dart';
 import '../import_table.dart';
 
 class Namer<T> {
   int index = 0;
@@ skipping 663 matching lines @@
       endLine();
     }
   }
 
   visitLibrary(Library node) {}
 
   visitField(Field node) {
     writeAnnotationList(node.annotations);
     writeIndentation();
     writeModifier(node.isStatic, 'static');
+    writeModifier(node.isCovariant, 'covariant');
     writeModifier(node.isFinal, 'final');
     writeModifier(node.isConst, 'const');
     // Only show implicit getter/setter modifiers in cases where they are
     // out of the ordinary.
     if (node.isStatic) {
       writeModifier(node.hasImplicitGetter, '[getter]');
       writeModifier(node.hasImplicitSetter, '[setter]');
     } else {
       writeModifier(!node.hasImplicitGetter, '[no-getter]');
       if (node.isFinal) {
@@ skipping 718 matching lines @@
       writeFunction(node.function, name: getVariableName(node.variable));
     } else {
       writeWord(getVariableName(node.variable));
       endLine('...;');
     }
   }
 
   void writeVariableDeclaration(VariableDeclaration node,
       {bool useVarKeyword: false}) {
     if (showOffsets) writeWord("[${node.fileOffset}]");
+    writeModifier(node.isCovariant, 'covariant');
     writeModifier(node.isFinal, 'final');
     writeModifier(node.isConst, 'const');
     if (node.type != null) {
       writeAnnotatedType(node.type, annotator?.annotateVariable(this, node));
     }
     if (useVarKeyword && !node.isFinal && !node.isConst && node.type == null) {
       writeWord('var');
     }
     writeWord(getVariableName(node));
     if (node.initializer != null) {
@@ skipping 238 matching lines @@
   }
   throw 'illegal ProcedureKind: $kind';
 }
 
 class ExpressionPrinter {
   final Printer writeer;
   final int minimumPrecedence;
 
   ExpressionPrinter(this.writeer, this.minimumPrecedence);
 }