dart2js: fix for https://github.com/dart-lang/sdk/issues/24412

pkg/compiler/lib/src/js_backend/codegen/glue.dart

 // Copyright (c) 2014, 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 code_generator_dependencies;
 
 import '../js_backend.dart';
 import '../../common/registry.dart' show
     Registry;
 import '../../compiler.dart' show
@@ skipping 243 matching lines @@
     return _backend.operatorEqHandlesNullArgument(element);
   }
 
   bool hasStrictSubtype(ClassElement element) {
     return _compiler.world.hasAnyStrictSubtype(element);
   }
 
   ClassElement get jsExtendableArrayClass => _backend.jsExtendableArrayClass;
   ClassElement get jsMutableArrayClass => _backend.jsMutableArrayClass;
 
+  ClassElement get jsPlainJavaScriptObjectClass =>
+      _backend.jsPlainJavaScriptObjectClass;
+  ClassElement get jsUnknownJavaScriptObjectClass =>
+      _backend.jsUnknownJavaScriptObjectClass;
+
   bool isStringClass(ClassElement classElement) =>
       classElement == _backend.jsStringClass ||
       classElement == _compiler.stringClass;
 
   bool isBoolClass(ClassElement classElement) =>
       classElement == _backend.jsBoolClass ||
       classElement == _compiler.boolClass;
 }