Cleanup 1: split parts into their own libraries, remove unused 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 '../common/names.dart' show
-    Identifiers;
 import '../compiler.dart' show
     Compiler;
 import '../core_types.dart' show
     CoreClasses;
 import '../dart_types.dart' show
     InterfaceType;
 import '../elements/elements.dart' show
     ClassElement,
     Element;
 
@@ skipping 627 matching lines @@
 
   BackendImpact get closure {
     if (_closure == null) {
       _closure = new BackendImpact(
           instantiatedClasses: [
             coreClasses.functionClass]);
     }
     return _closure;
   }
 }