Introduce Substitution class and Supertype class.

lib/binary/ast_to_binary.dart

Index: lib/binary/ast_to_binary.dart
diff --git a/lib/binary/ast_to_binary.dart b/lib/binary/ast_to_binary.dart
index 853a53a527acc69379cc2085ffcf38b31387b2b0..7f7c3927612ace3ea88863427d03b832618b37f4 100644
--- a/lib/binary/ast_to_binary.dart
+++ b/lib/binary/ast_to_binary.dart
@@ -870,6 +870,17 @@ class BinaryPrinter extends Visitor {
     }
   }
 
+  visitSupertype(Supertype node) {
+    if (node.typeArguments.isEmpty) {
+      writeByte(Tag.SimpleInterfaceType);
+      writeClassReference(node.classNode);
+    } else {
+      writeByte(Tag.InterfaceType);
+      writeClassReference(node.classNode);
+      writeNodeList(node.typeArguments);
+    }
+  }
+
   visitFunctionType(FunctionType node) {
     if (node.requiredParameterCount == node.positionalParameters.length &&
         node.typeParameters.isEmpty &&