Introduce Substitution class and Supertype class.

lib/visitor.dart

Index: lib/visitor.dart
diff --git a/lib/visitor.dart b/lib/visitor.dart
index c26efc6db55a0ab9eb7939444e044bbd915acde3..409c949a40b6253ee86651351d9410e5eb88801d 100644
--- a/lib/visitor.dart
+++ b/lib/visitor.dart
@@ -267,6 +267,7 @@ class Visitor<R> extends TreeVisitor<R>
   R visitProcedureReference(Procedure node) => defaultMemberReference(node);
 
   R visitName(Name node) => defaultNode(node);
+  R visitSupertype(Supertype node) => defaultNode(node);
 }
 
 class RecursiveVisitor<R> extends Visitor<R> {
@@ -306,6 +307,8 @@ class Transformer extends TreeVisitor<TreeNode> {
   /// 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;