Implement subtype relation for kernel based elements.

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

Index: pkg/compiler/lib/src/js_backend/backend_usage.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_usage.dart b/pkg/compiler/lib/src/js_backend/backend_usage.dart
index 0f1a3f3a51b92e8d5094a381a6ef8e3750b4715d..8d5f5460bc4b1b3ac7b2a0c11e99e935903ee170 100644
--- a/pkg/compiler/lib/src/js_backend/backend_usage.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_usage.dart
@@ -339,8 +339,13 @@ class BackendUsageImpl implements BackendUsage {
 
   @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;
 }