Remove direct access to enqueuer through use of WorldImpact

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 83 matching lines @@
         compiler.globalDependencies,
         backend,
         compiler.commonElements,
         compiler.cacheStrategy,
         'enqueuer from kernel');
     // TODO(johnniwinther): Store backend info separately. This replacement is
     // made to reset a field in [TypeVariableHandler] that prevents it from
     // enqueuing twice.
     backend.typeVariableHandler = new TypeVariableHandler(compiler);
 
-    backend.enqueueHelpers(enqueuer);
+    if (compiler.deferredLoadTask.isProgramSplit) {
+      enqueuer.applyImpact(backend.computeDeferredLoadingImpact());
+    }
+    enqueuer.applyImpact(backend.computeHelpersImpact());
+    enqueuer.applyImpact(enqueuer.nativeEnqueuer
+        .processNativeClasses(compiler.libraryLoader.libraries));
     enqueuer.applyImpact(
-        compiler.impactStrategy,
-        enqueuer.nativeEnqueuer
-            .processNativeClasses(compiler.libraryLoader.libraries));
-    enqueuer.applyImpact(compiler.impactStrategy,
-        backend.computeMainImpact(enqueuer, compiler.mainFunction));
+        backend.computeMainImpact(compiler.mainFunction, forResolution: true));
     enqueuer.forEach((work) {
       AstElement element = work.element;
       ResolutionImpact resolutionImpact = build(compiler, element.resolvedAst);
       WorldImpact worldImpact = compiler.backend.impactTransformer
           .transformResolutionImpact(enqueuer, resolutionImpact);
       enqueuer.registerProcessedElement(element);
-      enqueuer.applyImpact(compiler.impactStrategy, worldImpact,
-          impactSource: element);
+      enqueuer.applyImpact(worldImpact, impactSource: element);
     });
     ClosedWorld closedWorld =
         enqueuer.universe.openWorld.closeWorld(compiler.reporter);
 
     checkResolutionEnqueuers(compiler.enqueuer.resolution, enqueuer,
         typeEquivalence: (DartType a, DartType b) {
       return areTypesEquivalent(unalias(a), unalias(b));
     }, elementFilter: (Element element) {
       if (element is ConstructorElement && element.isRedirectingFactory) {
         // Redirecting factory constructors are skipped in kernel.
         return false;
       }
       if (element is ClassElement) {
         for (ConstructorElement constructor in element.constructors) {
           if (!constructor.isRedirectingFactory) {
             return true;
           }
         }
         // The class cannot itself be instantiated.
         return false;
       }
       return true;
     }, verbose: arguments.verbose);
     checkClosedWorlds(compiler.closedWorld, closedWorld,
         verbose: arguments.verbose);
   });
 }