dart2js cleanup: remove unused imports and variables

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 '../dart_types.dart';
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
+import '../dart_types.dart';
 import '../elements/elements.dart';
-import '../universe/call_structure.dart';
 import '../universe/selector.dart';
+import '../universe/use.dart';
 import '../universe/world_impact.dart';
-import '../universe/use.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 {
   final ObjectEncoder objectEncoder;
   final ListEncoder staticUses;
   final ListEncoder dynamicUses;
@@ skipping 155 matching lines @@
         constSymbolNames: constSymbolNames,
         constantLiterals: constantLiterals,
         dynamicUses: dynamicUses,
         features: features,
         listLiterals: listLiterals,
         mapLiterals: mapLiterals,
         staticUses: staticUses,
         typeUses: typeUses);
   }
 }