Use entities in NativeResolutionEnqueuer and make it reusable for kernel

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");
+  }
+}