Introduce Substitution class and Supertype class.

lib/visitor.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.visitor;
 
 import 'ast.dart';
 
 abstract class ExpressionVisitor<R> {
   R defaultExpression(Expression node) => null;
   R defaultBasicLiteral(BasicLiteral node) => defaultExpression(node);
@@ skipping 249 matching lines @@
   // Class references
   R visitClassReference(Class node) => null;
 
   // Member references
   R defaultMemberReference(Member node) => null;
   R visitFieldReference(Field node) => defaultMemberReference(node);
   R visitConstructorReference(Constructor node) => defaultMemberReference(node);
   R visitProcedureReference(Procedure node) => defaultMemberReference(node);
 
   R visitName(Name node) => defaultNode(node);
+  R visitSupertype(Supertype node) => defaultNode(node);
 }
 
 class RecursiveVisitor<R> extends Visitor<R> {
   R defaultNode(Node node) {
     node.visitChildren(this);
     return null;
   }
 }
 
 /// Visitor that recursively rewrites each node in tree.
@@ skipping 19 matching lines @@
 ///         return node;
 ///       }
 ///     }
 ///
 class Transformer extends TreeVisitor<TreeNode> {
   /// Replaces a use of a type.
   ///
   /// By default, recursion stops at this point.
   DartType visitDartType(DartType node) => node;
 
+  Supertype visitSupertype(Supertype node) => node;
+
   TreeNode defaultTreeNode(TreeNode node) {
     node.transformChildren(this);
     return node;
   }
 }