Further reduce use of Compiler.closedWorld.

pkg/compiler/lib/src/js_backend/backend.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 js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 3216 matching lines @@
   ClassElement get nullImplementation => helpers.jsNullClass;
   ClassElement get syncStarIterableImplementation => helpers.syncStarIterable;
   ClassElement get asyncFutureImplementation => helpers.futureImplementation;
   ClassElement get asyncStarStreamImplementation => helpers.controllerStream;
   ClassElement get functionImplementation => helpers.coreClasses.functionClass;
   ClassElement get indexableImplementation => helpers.jsIndexableClass;
   ClassElement get mutableIndexableImplementation =>
       helpers.jsMutableIndexableClass;
   ClassElement get indexingBehaviorImplementation =>
       helpers.jsIndexingBehaviorInterface;
+  ClassElement get interceptorImplementation => helpers.jsInterceptorClass;
 
   bool isDefaultEqualityImplementation(Element element) {
     assert(element.name == '==');
     ClassElement classElement = element.enclosingClass;
     return classElement == helpers.coreClasses.objectClass ||
         classElement == helpers.jsInterceptorClass ||
         classElement == helpers.jsNullClass;
   }
 
   @override
   bool isInterceptorClass(ClassElement cls) {
     return helpers.backend.isInterceptorClass(cls);
   }
 
   @override
   bool isNative(Element element) {
     return helpers.backend.isNative(element);
   }
 }