Split implementation of KernelToElementMapImpl

tests/compiler/dart2js/kernel/closed_world_from_dill_test.dart

 // Copyright (c) 2017, 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.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.closed_world_from_dill_test;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 121 matching lines @@
   }
   Expect.isFalse(compiler1.compilationFailed);
   ResolutionEnqueuer enqueuer1 = compiler1.enqueuer.resolution;
   ClosedWorld closedWorld1 = compiler1.resolutionWorldBuilder.closeWorld();
 
   Compiler compiler2 = await compileWithDill(
       entryPoint, const {}, [Flags.analyzeOnly, Flags.enableAssertMessage],
       printSteps: true);
 
   KernelFrontEndStrategy frontendStrategy = compiler2.frontendStrategy;
-  KernelToElementMapForBuilding elementMap = frontendStrategy.elementMap;
+  KernelToElementMap elementMap = frontendStrategy.elementMap;
 
   Expect.isFalse(compiler2.compilationFailed);
 
   KernelEquivalence equivalence = new KernelEquivalence(elementMap);
 
   ResolutionEnqueuer enqueuer2 = compiler2.enqueuer.resolution;
   ClosedWorld closedWorld2 = compiler2.resolutionWorldBuilder.closeWorld();
 
   checkBackendUsage(closedWorld1.backendUsage, closedWorld2.backendUsage,
       equivalence.defaultStrategy);
 
   checkResolutionEnqueuers(closedWorld1.backendUsage, closedWorld2.backendUsage,
       enqueuer1, enqueuer2,
       elementEquivalence: (a, b) => equivalence.entityEquivalence(a, b),
       typeEquivalence: (DartType a, DartType b) {
         return equivalence.typeEquivalence(unalias(a), b);
       },
       elementFilter: elementFilter,
       verbose: arguments.verbose);
 
   checkClosedWorlds(closedWorld1, closedWorld2,
       strategy: equivalence.defaultStrategy, verbose: arguments.verbose);
 
   return ResultKind.success;
 }