Introduce Substitution class and Supertype class.

test/type_parser.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.
 // A very simple parser for a subset of DartTypes for use in testing type
 // algebra.
 library kernel.test.type_parser;
 
 import 'package:kernel/kernel.dart';
 import 'package:kernel/text/ast_to_text.dart';
 
@@ skipping 277 matching lines @@
     dummyLibrary.addClass(class_);
     return class_;
   }
 
   void clearTypeParameters() {
     typeParameters.clear();
   }
 
   DartType parse(String type) => parseDartType(type, lookup);
 
+  Supertype parseSuper(String type) {
+    InterfaceType interfaceType = parse(type);
+    return new Supertype(interfaceType.classNode, interfaceType.typeArguments);
+  }
+
   DartType parseFresh(String type) {
     clearTypeParameters();
     return parse(type);
   }
 
   TypeParameter getTypeParameter(String name) {
     if (name.length != 1) throw 'Type parameter names must have length 1';
     return lookup(name);
   }
 }
 
 void main(List<String> args) {
   if (args.length != 1) {
     print('Usage: type_parser TYPE');
   }
   var environment = new LazyTypeEnvironment();
   var type = parseDartType(args[0], environment.lookup);
   var buffer = new StringBuffer();
   new Printer(buffer).writeType(type);
   print(buffer);
 }