Remove direct access to enqueuer through use of WorldImpact

tests/compiler/dart2js/compiler_helper.dart

 // Copyright (c) 2012, 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 compiler_helper;
 
 import 'dart:async';
 import "package:expect/expect.dart";
 
 import 'package:compiler/src/elements/elements.dart' as lego;
@@ skipping 54 matching lines @@
         disableTypeInference: true,
         enableMinification: minify,
         disableInlining: disableInlining,
         trustJSInteropTypeAnnotations: trustJSInteropTypeAnnotations,
         outputProvider: outputCollector);
     await compiler.init();
     compiler.parseScript(code);
     lego.Element element = compiler.mainApp.find(entry);
     if (element == null) return null;
     compiler.phase = Compiler.PHASE_RESOLVING;
-    compiler.backend.enqueueHelpers(compiler.enqueuer.resolution);
+    compiler.enqueuer.resolution
+        .applyImpact(compiler.backend.computeHelpersImpact());
     compiler.processQueue(compiler.enqueuer.resolution, element);
     compiler.openWorld.closeWorld(compiler.reporter);
     compiler.backend.onResolutionComplete();
     ResolutionWorkItem resolutionWork = new ResolutionWorkItem(element);
     resolutionWork.run(compiler, compiler.enqueuer.resolution);
     CodegenWorkItem work = new CodegenWorkItem(compiler, element);
     compiler.phase = Compiler.PHASE_COMPILING;
     work.run(compiler, compiler.enqueuer.codegen);
     js.JavaScriptBackend backend = compiler.backend;
     String generated = backend.getGeneratedCode(element);
@@ skipping 194 matching lines @@
     regexp = regexp.replaceAll(xRe, '(?:$anyIdentifier)');
     final spaceRe = new RegExp('\\s+');
     regexp = regexp.replaceAll(spaceRe, '(?:\\s*)');
     if (shouldMatch) {
       Expect.isTrue(new RegExp(regexp).hasMatch(generated));
     } else {
       Expect.isFalse(new RegExp(regexp).hasMatch(generated));
     }
   });
 }