Remove WorldImpact from caches when no longer needed.

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

 // Copyright (c) 2012, 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.
 
 library js_backend;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 29 matching lines @@
 import '../compiler.dart' show
     Compiler;
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 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;
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer;
 import '../io/code_output.dart';
 import '../io/source_information.dart' show
     SourceInformationStrategy,
     useNewSourceInfo;
@@ skipping 25 matching lines @@
     Selector,
     SelectorKind;
 import '../universe/universe.dart';
 import '../universe/use.dart' show
     DynamicUse,
     StaticUse,
     StaticUseKind,
     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
     ClassWorld;
 
 import 'backend_helpers.dart';
 import 'backend_impact.dart';
 import 'codegen/task.dart';
 import 'constant_system_javascript.dart';
 import 'patch_resolver.dart';
 import 'js_interop_analysis.dart' show JsInteropAnalysis;
 import 'lookup_map_analysis.dart' show LookupMapAnalysis;
 
 part 'backend.dart';
 part 'checked_mode_helpers.dart';
 part 'constant_emitter.dart';
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
 part 'frequency_namer.dart';
 part 'field_naming_mixin.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';