Fix hints reported by analyzer.

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

 // Copyright (c) 2015, 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.js_helpers.impact;
 
 import '../common/names.dart';
 import '../compiler.dart' show Compiler;
 import '../core_types.dart' show CommonElements;
 import '../elements/resolution_types.dart' show ResolutionInterfaceType;
 import '../elements/elements.dart' show ClassElement, Element;
 import '../universe/selector.dart';
 import '../util/enumset.dart';
 import 'backend_helpers.dart';
-import 'constant_system_javascript.dart';
 import 'js_backend.dart';
 
 /// Backend specific features required by a backend impact.
 enum BackendFeature {
   needToInitializeIsolateAffinityTag,
   needToInitializeDispatchProperty,
 }
 
 /// A set of JavaScript backend dependencies.
 class BackendImpact {
@@ skipping 703 matching lines @@
 
   BackendImpact _staticClosure;
 
   /// Backend impact for performing closurization of a top-level or static
   /// function.
   BackendImpact get staticClosure {
     return _staticClosure ??=
         new BackendImpact(globalClasses: [helpers.closureClass]);
   }
 }