Introduce Substitution class and Supertype class.

lib/transformations/treeshaker.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.tree_shaker;
 
 import '../ast.dart';
 import '../class_hierarchy.dart';
 import '../core_types.dart';
 
@@ skipping 644 matching lines @@
 
   Class visitClass(Class node) {
     switch (shaker.getClassRetention(node)) {
       case ClassRetention.None:
         return null; // Remove the class.
 
       case ClassRetention.Namespace:
         // The class is only a namespace for static members.  Remove its
         // hierarchy information.   This is mandatory, since these references
         // might otherwise become dangling.
-        node.supertype = shaker.coreTypes.objectClass.rawType;
+        node.supertype = shaker.coreTypes.objectClass.asRawSupertype;
         node.implementedTypes.clear();
         node.typeParameters.clear();
         // Mixin applications cannot have static members.
         assert(node.mixedInType == null);
         // Unused members will be removed below.
         break;
 
       case ClassRetention.Hierarchy:
       case ClassRetention.Instance:
       case ClassRetention.ExternalInstance:
@@ skipping 92 matching lines @@
     if (classNode.typeParameters.isEmpty) return false;
     CoreTypes coreTypes = shaker.coreTypes;
     return classNode == coreTypes.iteratorClass ||
         classNode == coreTypes.iterableClass ||
         classNode == coreTypes.futureClass ||
         classNode == coreTypes.streamClass ||
         classNode == coreTypes.listClass ||
         classNode == coreTypes.mapClass;
   }
 }