Handle symbol literals in closed_world2_test

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

 // Copyright (c) 2017, 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 '../common.dart';
 import '../common_elements.dart';
 import '../elements/elements.dart' show Element;
 import '../elements/entities.dart';
 import '../elements/types.dart';
 import '../util/util.dart' show Setlet;
@@ skipping 148 matching lines @@
   }
 
   bool _isValidEntity(Entity element) {
     if (element is ConstructorEntity &&
         (element == _commonElements.streamIteratorConstructor ||
             _commonElements.isSymbolConstructor(element) ||
             _commonElements.isSymbolValidatedConstructor(element) ||
             element == _commonElements.syncCompleterConstructor)) {
       // TODO(johnniwinther): These are valid but we could be more precise.
       return true;
-    } else if (element == _commonElements.symbolClass ||
+    } else if (element == _commonElements.symbolImplementationClass ||
         element == _commonElements.objectNoSuchMethod) {
       // TODO(johnniwinther): These are valid but we could be more precise.
       return true;
     } else if (element == _commonElements.listClass ||
         element == _commonElements.mapLiteralClass ||
         element == _commonElements.functionClass ||
         element == _commonElements.stringClass) {
       // TODO(johnniwinther): Avoid these.
       return true;
     } else if (element == _commonElements.genericNoSuchMethod ||
@@ skipping 162 matching lines @@
       _globalFunctionDependencies ?? const <FunctionEntity>[];
 
   @override
   Iterable<ClassEntity> get globalClassDependencies =>
       _globalClassDependencies ?? const <ClassEntity>[];
 
   Iterable<FunctionEntity> get helperFunctionsUsed => _helperFunctionsUsed;
 
   Iterable<ClassEntity> get helperClassesUsed => _helperClassesUsed;
 }