Revert "Remove WorldImpact from caches when no longer needed."

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

Index: pkg/compiler/lib/src/js_backend/js_backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/js_backend.dart b/pkg/compiler/lib/src/js_backend/js_backend.dart
index 99d383c04e3abf2f256f7c111a7fe5ffab8ee67f..9c69c453fd34f26a33dc772db8c9f90da830294c 100644
--- a/pkg/compiler/lib/src/js_backend/js_backend.dart
+++ b/pkg/compiler/lib/src/js_backend/js_backend.dart
@@ -47,10 +47,6 @@ import '../core_types.dart' show
     CoreClasses,
     CoreTypes;
 import '../dart_types.dart';
-import '../deferred_load.dart' show
-    DeferredLoadTask;
-import '../dump_info.dart' show
-    DumpInfoTask;
 import '../elements/elements.dart';
 import '../elements/visitor.dart' show
     BaseElementVisitor;
@@ -96,11 +92,8 @@ import '../universe/use.dart' show
     TypeUse,
     TypeUseKind;
 import '../universe/world_impact.dart' show
-    ImpactStrategy,
-    ImpactUseCase,
     TransformedWorldImpact,
-    WorldImpact,
-    WorldImpactVisitor;
+    WorldImpact;
 import '../util/characters.dart';
 import '../util/util.dart';
 import '../world.dart' show