Add kernel/closed_world2_test

pkg/compiler/lib/src/kernel/native_class_resolver.dart

Index: pkg/compiler/lib/src/kernel/native_class_resolver.dart
diff --git a/pkg/compiler/lib/src/kernel/native_class_resolver.dart b/pkg/compiler/lib/src/kernel/native_class_resolver.dart
index b86ae5be10e8b9d73cfeda0b3c1aaafca53b8b58..f02d48bd0092af5edbc2949b6ec397920410c8f0 100644
--- a/pkg/compiler/lib/src/kernel/native_class_resolver.dart
+++ b/pkg/compiler/lib/src/kernel/native_class_resolver.dart
@@ -12,7 +12,7 @@ class KernelNativeClassResolver implements NativeClassResolver {
 
   Iterable<ClassEntity> computeNativeClasses(
       Iterable<LibraryEntity> libraries) {
-    throw new UnimplementedError(
-        "KernelNativeClassResolver.computeNativeClasses");
+    // TODO(johnniwinther): Implement this.
+    return const <ClassEntity>[];
   }
 }