Move WorldImpact to universe/world_impact.dart

pkg/compiler/lib/src/dart_backend/dart_backend.dart

Index: pkg/compiler/lib/src/dart_backend/dart_backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/dart_backend.dart b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
index 57b7af5036b561cc6c3d83e7ff5b9d7b0ba680a4..6add624a251ee6aaafc6fa284c338a81d5455848 100644
--- a/pkg/compiler/lib/src/dart_backend/dart_backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
@@ -36,8 +36,7 @@ import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../enqueue.dart' show
     Enqueuer,
-    ResolutionEnqueuer,
-    WorldImpact;
+    ResolutionEnqueuer;
 import '../library_loader.dart' show
     LoadedLibraries;
 import '../mirror_renamer/mirror_renamer.dart';
@@ -51,6 +50,8 @@ import '../universe/selector.dart' show
     Selector;
 import '../universe/universe.dart' show
     UniverseSelector;
+import '../universe/world_impact.dart' show
+    WorldImpact;
 import '../util/util.dart';
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;