Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/serialization/impact_serialization.dart

 // Copyright (c) 2016, 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 dart2js.serialization.impact;
 
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../universe/selector.dart';
+import '../universe/feature.dart';
 import '../universe/use.dart';
 import '../universe/world_impact.dart';
 import '../util/enumset.dart';
 import 'keys.dart';
 import 'serialization.dart';
 import 'serialization_util.dart';
 
 /// Visitor that serializes a [ResolutionImpact] object using an
 /// [ObjectEncoder].
 class ImpactSerializer implements WorldImpactVisitor {
@@ skipping 186 matching lines @@
         constantLiterals: constantLiterals,
         dynamicUses: dynamicUses,
         features: features,
         listLiterals: listLiterals,
         mapLiterals: mapLiterals,
         staticUses: staticUses,
         typeUses: typeUses,
         nativeData: nativeData);
   }
 }