Add kernel/closed_world2_test

pkg/compiler/lib/src/kernel/native_class_resolver.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.
 
 part of dart2js.kernel.world_builder;
 
 /// Interface for computing all native classes in a set of libraries.
 class KernelNativeClassResolver implements NativeClassResolver {
   final KernelWorldBuilder _worldBuilder;
 
   KernelNativeClassResolver(this._worldBuilder);
 
   Iterable<ClassEntity> computeNativeClasses(
       Iterable<LibraryEntity> libraries) {
-    throw new UnimplementedError(
-        "KernelNativeClassResolver.computeNativeClasses");
+    // TODO(johnniwinther): Implement this.
+    return const <ClassEntity>[];
   }
 }