Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

tests/compiler/dart2js/kernel/closed_world_test.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.
 
 // Tests that the closed world computed from [WorldImpact]s derived from kernel
 // is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world_test;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
@@ skipping 203 matching lines @@
       customElementsResolutionAnalysis,
       lookupMapResolutionAnalysis,
       mirrorsResolutionAnalysis,
       new TypeVariableResolutionAnalysis(
           elementEnvironment, impacts, backendUsageBuilder),
       nativeResolutionEnqueuer,
       deferredLoadTask);
 }
 
 ClosedWorld computeClosedWorld(Compiler compiler, ResolutionEnqueuer enqueuer) {
-  JavaScriptBackend backend = compiler.backend;
-
   enqueuer.open(const ImpactStrategy(), compiler.mainFunction,
       compiler.libraryLoader.libraries);
   enqueuer.forEach((work) {
     MemberElement element = work.element;
     ResolutionImpact resolutionImpact = build(compiler, element.resolvedAst);
     WorldImpact worldImpact = compiler.backend.impactTransformer
         .transformResolutionImpact(enqueuer, resolutionImpact);
     enqueuer.applyImpact(worldImpact, impactSource: element);
   });
   return enqueuer.worldBuilder.closeWorld(compiler.reporter);
 }