Use JsStrategy, remove KernelBackendStrategy

tests/compiler/dart2js/kernel/closed_world2_test.dart

Index: tests/compiler/dart2js/kernel/closed_world2_test.dart
diff --git a/tests/compiler/dart2js/kernel/closed_world2_test.dart b/tests/compiler/dart2js/kernel/closed_world2_test.dart
index 219ce62bbc023aee225063f5a9afc88d54e6361c..952479c3d49100cb37cee87a5a99dd8821104d9e 100644
--- a/tests/compiler/dart2js/kernel/closed_world2_test.dart
+++ b/tests/compiler/dart2js/kernel/closed_world2_test.dart
@@ -16,7 +16,7 @@ import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/types.dart';
 import 'package:compiler/src/enqueue.dart';
 import 'package:compiler/src/js_backend/backend_usage.dart';
-import 'package:compiler/src/kernel/element_map_impl.dart';
+import 'package:compiler/src/kernel/element_map.dart';
 import 'package:compiler/src/kernel/kernel_strategy.dart';
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/universe/world_builder.dart';
@@ -128,7 +128,7 @@ Future<ResultKind> mainInternal(List<String> args,
 
   Compiler compiler2 = compilers.b;
   KernelFrontEndStrategy frontendStrategy = compiler2.frontendStrategy;
-  KernelToElementMapImpl elementMap = frontendStrategy.elementMap;
+  KernelToElementMapForImpact elementMap = frontendStrategy.elementMap;
   Expect.isFalse(compiler2.compilationFailed);
 
   KernelEquivalence equivalence = new KernelEquivalence(elementMap);