Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/kernel/closed_world_test.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.
 
 // Tests that the closed world computed from [WorldImpact]s derived from kernel
 // is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world_test;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 118 matching lines @@
         for (ConstructorElement constructor in element.constructors) {
           if (!constructor.isRedirectingFactory) {
             return true;
           }
         }
         // The class cannot itself be instantiated.
         return false;
       }
       return true;
     }, verbose: arguments.verbose);
-    checkClosedWorlds(compiler.closedWorld, closedWorld,
+    checkClosedWorlds(compiler.resolverWorld.closedWorldForTesting, closedWorld,
         verbose: arguments.verbose);
   });
 }