Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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

 // Copyright (c) 2013, 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.
 
 import '../compiler.dart' show Compiler;
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../ssa/codegen.dart' show SsaCodeGenerator;
 import '../ssa/nodes.dart' show HTypeConversion;
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/use.dart' show StaticUse;
 import 'backend.dart';
 
 class CheckedModeHelper {
   final String name;
 
   const CheckedModeHelper(String this.name);
 
   StaticUse getStaticUse(Compiler compiler) {
     JavaScriptBackend backend = compiler.backend;
     // TODO(johnniwinther): Refactor this to avoid looking up directly in the
     // js helper library but instead access helpers directly on backend helpers.
-    MethodElement method = backend.helpers.jsHelperLibrary.find(name);
+    LibraryElement jsHelperLibrary = backend.helpers.jsHelperLibrary;
+    MethodElement method = jsHelperLibrary.find(name);
     return new StaticUse.staticInvoke(method, callStructure);
   }
 
   CallStructure get callStructure => CallStructure.ONE_ARG;
 
   jsAst.Expression generateCall(
       SsaCodeGenerator codegen, HTypeConversion node) {
     StaticUse staticUse = getStaticUse(codegen.compiler);
     codegen.registry.registerStaticUse(staticUse);
     List<jsAst.Expression> arguments = <jsAst.Expression>[];
@@ skipping 97 matching lines @@
     ResolutionDartType type = node.typeExpression;
     Element element = type.element;
     jsAst.Name isField = codegen.backend.namer.operatorIs(element);
     arguments.add(js.quoteName(isField));
     codegen.use(node.typeRepresentation);
     arguments.add(codegen.pop());
     jsAst.Name asField = codegen.backend.namer.substitutionName(element);
     arguments.add(js.quoteName(asField));
   }
 }