pkg/compiler: fix imports

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 '../compiler.dart' show Compiler;
 import '../core_types.dart' show CoreClasses;
 import '../dart_types.dart' show InterfaceType;
 import '../elements/elements.dart' show ClassElement, Element;
-
 import 'backend_helpers.dart';
 import 'constant_system_javascript.dart';
 import 'js_backend.dart';
 
 /// A set of JavaScript backend dependencies.
 class BackendImpact {
   final List<Element> staticUses;
   final List<InterfaceType> instantiatedTypes;
   final List<ClassElement> instantiatedClasses;
   final List<BackendImpact> otherImpacts;
@@ skipping 577 matching lines @@
   BackendImpact _closure;
 
   BackendImpact get closure {
     if (_closure == null) {
       _closure =
           new BackendImpact(instantiatedClasses: [coreClasses.functionClass]);
     }
     return _closure;
   }
 }