Move main and isolate handling to enqueuer listeners.

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 118 matching lines @@
   JavaScriptBackend backend = compiler.backend;
 
   // 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);
 
   if (compiler.deferredLoadTask.isProgramSplit) {
     enqueuer.applyImpact(backend.computeDeferredLoadingImpact());
   }
-  enqueuer.applyImpact(backend.computeHelpersImpact());
-  enqueuer.applyImpact(enqueuer.nativeEnqueuer
-      .processNativeClasses(compiler.libraryLoader.libraries));
-  enqueuer.applyImpact(
-      backend.computeMainImpact(compiler.mainFunction, forResolution: true));
+  enqueuer.open(const ImpactStrategy(), compiler.mainFunction,
+      compiler.libraryLoader.libraries);
   enqueuer.forEach((work) {
     AstElement 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);
 }