Implement subtype relation for kernel based elements.

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 321 matching lines @@
     return _helperFunctionsUsed.contains(element);
   }
 
   @override
   bool isFieldUsedByBackend(FieldEntity element) {
     return _helperClassesUsed.contains(element.enclosingClass);
   }
 
   @override
   Iterable<FunctionEntity> get globalFunctionDependencies =>
-      _globalFunctionDependencies;
+      _globalFunctionDependencies ?? const <FunctionEntity>[];
 
   @override
-  Iterable<ClassEntity> get globalClassDependencies => _globalClassDependencies;
+  Iterable<ClassEntity> get globalClassDependencies =>
+      _globalClassDependencies ?? const <ClassEntity>[];
+
+  Iterable<FunctionEntity> get helperFunctionsUsed => _helperFunctionsUsed;
+
+  Iterable<ClassEntity> get helperClassesUsed => _helperClassesUsed;
 }