Add ClassSet to support ClassWorld.strictSubtypesOf

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import '../compiler.dart' show
     Compiler;
 import '../constants/constant_constructors.dart';
 import '../constants/constructors.dart';
@@ skipping 2345 matching lines @@
 
   bool get isObject {
     assert(invariant(this, isResolved,
         message: "isObject has not been computed for $this."));
     return supertype == null;
   }
 
   void ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveClass(this);
+      compiler.world.registerClass(this);
     }
   }
 
   void setDefaultConstructor(FunctionElement constructor, Compiler compiler);
 
   void addBackendMember(Element member) {
     // TODO(ngeoffray): Deprecate this method.
     assert(member.isGenerativeConstructorBody);
     backendMembers = backendMembers.prepend(member);
   }
@@ skipping 538 matching lines @@
   AstElement get definingElement;
 
   bool get hasResolvedAst => definingElement.hasTreeElements;
 
   ResolvedAst get resolvedAst {
     return new ResolvedAst(declaration,
         definingElement.node, definingElement.treeElements);
   }
 
 }