Create SsaBuilderTask stub for KernelBackendStrategy

pkg/compiler/lib/src/backend_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.backend_strategy;
 
 import 'enqueue.dart';
+import 'io/source_information.dart';
+import 'js_backend/js_backend.dart';
 import 'js_backend/native_data.dart';
 import 'js_emitter/sorter.dart';
+import 'ssa/ssa.dart';
 import 'universe/world_builder.dart';
 import 'world.dart';
 
 /// Strategy pattern that defines the element model used in type inference
 /// and code generation.
 abstract class BackendStrategy {
   /// Create the [ClosedWorldRefiner] for [closedWorld].
   ClosedWorldRefiner createClosedWorldRefiner(ClosedWorld closedWorld);
 
   /// Create closure classes for local functions.
   void convertClosures(ClosedWorldRefiner closedWorldRefiner);
 
   /// The [Sorter] used for sorting elements in the generated code.
   Sorter get sorter;
 
   /// Creates the [CodegenWorldBuilder] used by the codegen enqueuer.
   CodegenWorldBuilder createCodegenWorldBuilder(
       NativeBasicData nativeBasicData,
       ClosedWorld closedWorld,
       SelectorConstraintsStrategy selectorConstraintsStrategy);
 
   /// Creates the [WorkItemBuilder] used by the codegen enqueuer.
   WorkItemBuilder createCodegenWorkItemBuilder(ClosedWorld closedWorld);
+
+  /// Creates the [SsaBuilderTask] used for the element model.
+  SsaBuilderTask createSsaBuilderTask(JavaScriptBackend backend,
+      SourceInformationStrategy sourceInformationStrategy);
 }