Use JsStrategy, remove KernelBackendStrategy

tests/compiler/dart2js/kernel/compiler_helper.dart

Index: tests/compiler/dart2js/kernel/compiler_helper.dart
diff --git a/tests/compiler/dart2js/kernel/compiler_helper.dart b/tests/compiler/dart2js/kernel/compiler_helper.dart
index bea95fa6639836a1aff9e21f19ed168810017445..1ce22d8cca88dafc80ebd672ce24f904e4fbd9f7 100644
--- a/tests/compiler/dart2js/kernel/compiler_helper.dart
+++ b/tests/compiler/dart2js/kernel/compiler_helper.dart
@@ -16,7 +16,7 @@ import 'package:compiler/src/common/names.dart';
 import 'package:compiler/src/common/tasks.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.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/library_loader.dart';
 import 'package:compiler/src/universe/world_builder.dart';
@@ -70,7 +70,7 @@ Future<List<CompileFunction>> compileMultiple(List<String> sources) async {
       ElementResolutionWorldBuilder.useInstantiationMap = true;
       compiler2.resolution.retainCachesForTesting = true;
       KernelFrontEndStrategy frontendStrategy = compiler2.frontendStrategy;
-      KernelToElementMapImpl elementMap = frontendStrategy.elementMap;
+      KernelToElementMapForImpact elementMap = frontendStrategy.elementMap;
       ir.Program program = new ir.Program(
           libraries:
               compiler.backend.kernelTask.kernel.libraryDependencies(uri));
@@ -116,7 +116,7 @@ Future<Pair<Compiler, Compiler>> analyzeOnly(
   ElementResolutionWorldBuilder.useInstantiationMap = true;
   compiler2.resolution.retainCachesForTesting = true;
   KernelFrontEndStrategy frontendStrategy = compiler2.frontendStrategy;
-  KernelToElementMapImpl elementMap = frontendStrategy.elementMap;
+  KernelToElementMapForImpact elementMap = frontendStrategy.elementMap;
   compiler2.libraryLoader = new MemoryDillLibraryLoaderTask(
       elementMap,
       compiler2.reporter,
@@ -129,7 +129,7 @@ Future<Pair<Compiler, Compiler>> analyzeOnly(
 class MemoryDillLibraryLoaderTask extends DillLibraryLoaderTask {
   final ir.Program program;
 
-  MemoryDillLibraryLoaderTask(KernelToElementMapImpl elementMap,
+  MemoryDillLibraryLoaderTask(KernelToElementMapForImpact elementMap,
       DiagnosticReporter reporter, Measurer measurer, this.program)
       : super(elementMap, null, null, reporter, measurer);