Use JsStrategy, remove KernelBackendStrategy

pkg/compiler/lib/src/kernel/kernel_strategy.dart

Index: pkg/compiler/lib/src/kernel/kernel_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
index 12896e03ab0c2f1b438fe7eb0766161e01023bde..3fb2ae6bb829534bc29e96231364ca291c34b4a9 100644
--- a/pkg/compiler/lib/src/kernel/kernel_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_strategy.dart
@@ -37,7 +37,6 @@ import '../universe/world_impact.dart';
 import '../world.dart';
 import 'element_map.dart';
 import 'element_map_impl.dart';
-import 'kernel_backend_strategy.dart';
 
 /// Front end strategy that loads '.dill' files and builds a resolved element
 /// model from kernel IR nodes.
@@ -49,9 +48,8 @@ class KernelFrontEndStrategy extends FrontendStrategyBase {
 
   KernelFrontEndStrategy(
       this._options, DiagnosticReporter reporter, env.Environment environment)
-      : _elementMap = useJsStrategyForTesting
-            ? new KernelToElementMapForImpactImpl2(reporter, environment)
-            : new KernelToElementMapImpl(reporter, environment);
+      : _elementMap =
+            new KernelToElementMapForImpactImpl(reporter, environment);
 
   @override
   LibraryLoaderTask createLibraryLoader(