Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/js_backend/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.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common/backend_api.dart'
     show Backend, ImpactTransformer, ForeignResolver, NativeRegistry;
 import '../common/codegen.dart' show CodegenImpact, CodegenWorkItem;
 import '../common/names.dart' show Identifiers, Selectors, Uris;
 import '../common/registry.dart' show EagerRegistry, Registry;
 import '../common/resolution.dart'
     show
-        Feature,
         Frontend,
-        ListLiteralUse,
-        MapLiteralUse,
         Resolution,
         ResolutionImpact;
 import '../common/tasks.dart' show CompilerTask;
 import '../common/work.dart' show ItemCompilationContext;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../core_types.dart' show CoreClasses, CoreTypes;
 import '../dart_types.dart';
@@ skipping 14 matching lines @@
 import '../native/native.dart' as native;
 import '../ssa/builder.dart' show SsaFunctionCompiler;
 import '../ssa/nodes.dart' show HInstruction;
 import '../tree/tree.dart';
 import '../types/types.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector, SelectorKind;
 import '../universe/universe.dart';
 import '../universe/use.dart'
     show DynamicUse, StaticUse, StaticUseKind, TypeUse, TypeUseKind;
+import '../universe/feature.dart';
 import '../universe/world_impact.dart'
     show
         ImpactStrategy,
         ImpactUseCase,
         TransformedWorldImpact,
         WorldImpact,
         WorldImpactVisitor;
 import '../util/util.dart';
 import '../world.dart' show ClassWorld;
 import 'backend_helpers.dart';
@@ skipping 3128 matching lines @@
 
   @override
   void onImpactUsed(ImpactUseCase impactUse) {
     if (impactUse == DeferredLoadTask.IMPACT_USE && !supportSerialization) {
       // TODO(johnniwinther): Allow emptying when serialization has been
       // performed.
       resolution.emptyCache();
     }
   }
 }