Create SsaBuilderTask stub for KernelBackendStrategy

pkg/compiler/lib/src/js_backend/element_strategy.dart

Index: pkg/compiler/lib/src/js_backend/element_strategy.dart
diff --git a/pkg/compiler/lib/src/js_backend/element_strategy.dart b/pkg/compiler/lib/src/js_backend/element_strategy.dart
index 2fb66213046ec6d5a05e6d6cf3e18b2389269ecd..5001724f086a73daaba2c9b547416ed2d521d82e 100644
--- a/pkg/compiler/lib/src/js_backend/element_strategy.dart
+++ b/pkg/compiler/lib/src/js_backend/element_strategy.dart
@@ -11,9 +11,13 @@ 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';
@@ -53,6 +57,14 @@ class ElementBackendStrategy implements BackendStrategy {
     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