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 9c69c453fd34f26a33dc772db8c9f90da830294c..c382e54fe184f3b18011a663fe0d9657604c0cb3 100644
--- a/pkg/compiler/lib/src/js_backend/js_backend.dart
+++ b/pkg/compiler/lib/src/js_backend/js_backend.dart
@@ -47,6 +47,10 @@ 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;
@@ -92,8 +96,11 @@ import '../universe/use.dart' show
     TypeUse,
     TypeUseKind;
 import '../universe/world_impact.dart' show
+    ImpactStrategy,
+    ImpactUse,
     TransformedWorldImpact,
-    WorldImpact;
+    WorldImpact,
+    WorldImpactVisitor;
 import '../util/characters.dart';
 import '../util/util.dart';
 import '../world.dart' show