Create SsaBuilderTask stub for KernelBackendStrategy

pkg/compiler/lib/src/js_backend/element_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.js_backend.element_strategy;
 
 import '../backend_strategy.dart';
 import '../common.dart';
 import '../common/codegen.dart';
 import '../common/work.dart';
 import '../compiler.dart';
 import '../elements/elements.dart';
 import '../enqueue.dart';
+import '../io/source_information.dart';
 import '../js_backend/backend.dart';
 import '../js_backend/native_data.dart';
 import '../js_emitter/sorter.dart';
+import '../ssa/builder.dart';
+import '../ssa/builder_kernel.dart';
+import '../ssa/ssa.dart';
 import '../options.dart';
 import '../universe/world_builder.dart';
 import '../universe/world_impact.dart';
 import '../world.dart';
 
 /// Strategy for using the [Element] model from the resolver as the backend
 /// model.
 class ElementBackendStrategy implements BackendStrategy {
   final Compiler _compiler;
 
@@ skipping 19 matching lines @@
         closedWorld,
         _compiler.backend.constants,
         selectorConstraintsStrategy);
   }
 
   @override
   WorkItemBuilder createCodegenWorkItemBuilder(ClosedWorld closedWorld) {
     return new ElementCodegenWorkItemBuilder(
         _compiler.backend, closedWorld, _compiler.options);
   }
+
+  @override
+  SsaBuilderTask createSsaBuilderTask(JavaScriptBackend backend,
+      SourceInformationStrategy sourceInformationStrategy) {
+    return _compiler.options.useKernel
+        ? new SsaAstKernelBuilderTask(backend, sourceInformationStrategy)
+        : new SsaAstBuilderTask(backend, sourceInformationStrategy);
+  }
 }
 
 /// Builder that creates the work item necessary for the code generation of a
 /// [MemberElement].
 class ElementCodegenWorkItemBuilder extends WorkItemBuilder {
   final JavaScriptBackend _backend;
   final ClosedWorld _closedWorld;
   final CompilerOptions _options;
 
   ElementCodegenWorkItemBuilder(
@@ skipping 42 matching lines @@
 
   MemberElement get element => resolvedAst.element;
 
   WorldImpact run() {
     registry = new CodegenRegistry(element);
     return _backend.codegen(this, _closedWorld);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }