Add BackendStrategy

pkg/compiler/lib/src/kernel/kernel_strategy.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.
 
 library dart2js.kernel.frontend_strategy;
 
 import '../closure.dart';
+import '../backend_strategy.dart';
 import '../common.dart';
 import '../common_elements.dart';
 import '../common/backend_api.dart';
 import '../common/resolution.dart';
 import '../common/tasks.dart';
 import '../common/work.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../elements/types.dart';
 import '../environment.dart' as env;
 import '../enqueue.dart';
 import '../frontend_strategy.dart';
 import '../js_backend/backend.dart';
 import '../js_backend/backend_usage.dart';
 import '../js_backend/custom_elements_analysis.dart';
 import '../js_backend/mirrors_analysis.dart';
 import '../js_backend/mirrors_data.dart';
 import '../js_backend/native_data.dart';
 import '../js_backend/no_such_method_registry.dart';
+import '../js_emitter/sorter.dart';
 import '../library_loader.dart';
 import '../native/resolver.dart';
 import '../serialization/task.dart';
 import '../patch_parser.dart';
 import '../resolved_uri_translator.dart';
 import '../universe/world_builder.dart';
 import '../universe/world_impact.dart';
 import '../world.dart';
 import 'element_map.dart';
 
@@ skipping 160 matching lines @@
   void onQueueEmpty(Enqueuer enqueuer, Iterable<ClassEntity> recentClasses) {}
 
   @override
   MirrorsCodegenAnalysis close() {
     throw new UnimplementedError('MirrorsResolutionAnalysisImpl.close');
   }
 
   @override
   void onResolutionComplete() {}
 }
+
+/// Backend strategy that uses the kernel elements as the backend model.
+// TODO(johnniwinther): Replace this with a strategy based on the J-element
+// model.
+class KernelBackendStrategy implements BackendStrategy {
+  @override
+  ClosedWorldRefiner createClosedWorldRefiner(KernelClosedWorld closedWorld) {
+    return closedWorld;
+  }
+
+  @override
+  Sorter get sorter =>
+      throw new UnimplementedError('KernelBackendStrategy.sorter');
+
+  @override
+  void convertClosures(ClosedWorldRefiner closedWorldRefiner) {
+    // TODO(johnniwinther,efortuna): Compute closure classes for kernel based
+    // elements.
+    throw new UnimplementedError('KernelBackendStrategy.createClosureClasses');
+  }
+}