Refactor BackendHelpers to be reusageable with KernelWorldBuilder

pkg/compiler/lib/src/js_backend/js_interop_analysis.dart

 // Copyright (c) 2015, 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.
 
 /// Analysis to determine how to generate code for typed JavaScript interop.
 library compiler.src.js_backend.js_interop_analysis;
 
 import '../common.dart';
 import '../constants/values.dart'
     show ConstantValue, ConstructedConstantValue, StringConstantValue;
@@ skipping 27 matching lines @@
   bool _inCodegen = false;
 
   JsInteropAnalysis(this.backend);
 
   BackendHelpers get helpers => backend.helpers;
 
   void onQueueClosed() {
     if (_inCodegen) return;
 
     if (helpers.jsAnnotationClass != null) {
-      nameField = helpers.jsAnnotationClass.lookupMember('name');
+      ClassElement cls = helpers.jsAnnotationClass;
+      nameField = cls.lookupMember('name');
       backend.compiler.libraryLoader.libraries
           .forEach(processJsInteropAnnotationsInLibrary);
     }
   }
 
   void onCodegenStart() {
     _inCodegen = true;
   }
 
   void processJsInteropAnnotation(Element e) {
@@ skipping 135 matching lines @@
   ResolutionFunctionType buildJsFunctionType() {
     // TODO(jacobr): consider using codegenWorldBuilder.isChecks to determine the
     // range of positional arguments that need to be supported by JavaScript
     // function types.
     return new ResolutionFunctionType.synthesized(
         const ResolutionDynamicType(),
         [],
         new List<ResolutionDartType>.filled(16, const ResolutionDynamicType()));
   }
 }