Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

pkg/compiler/lib/src/js_backend/enqueuer.dart

Index: pkg/compiler/lib/src/js_backend/enqueuer.dart
diff --git a/pkg/compiler/lib/src/js_backend/enqueuer.dart b/pkg/compiler/lib/src/js_backend/enqueuer.dart
index 4784df02cf69b31160c2a68d7a8e4d8efb5820ac..da79b0e16ef582625308d5c922b71de9e0549e35 100644
--- a/pkg/compiler/lib/src/js_backend/enqueuer.dart
+++ b/pkg/compiler/lib/src/js_backend/enqueuer.dart
@@ -86,7 +86,8 @@ class CodegenEnqueuer extends EnqueuerImpl {
   CodegenWorldBuilder get universe => _universe;
 
   // TODO(johnniwinther): Remove these hacks:
-  ClosedWorld get _world => _backend.compiler.closedWorld;
+  ClosedWorld get _world =>
+      _backend.compiler.resolverWorld.closedWorldForTesting;
   DumpInfoTask get _dumpInfoTask => _backend.compiler.dumpInfoTask;
 
   bool get queueIsEmpty => _queue.isEmpty;