Split implementation of KernelToElementMapImpl

tests/compiler/dart2js/js_model/model_test.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.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/compiler.dart';
+import 'package:compiler/src/kernel/kernel_backend_strategy.dart';
 import 'package:compiler/src/js_model/js_strategy.dart';
 import 'package:expect/expect.dart';
 import '../kernel/compiler_helper.dart';
 import '../serialization/helper.dart';
 
 const Map<String, String> SOURCE = const <String, String>{
   'main.dart': r'''
 main() {}
 '''
 };
 
 main(List<String> args) {
+  useJsStrategyForTesting = true;
   asyncTest(() async {
     await mainInternal(args);
   });
 }
 
 Future mainInternal(List<String> args,
     {bool skipWarnings: false, bool skipErrors: false}) async {
   Arguments arguments = new Arguments.from(args);
   Uri entryPoint;
   Map<String, String> memorySourceFiles;
   if (arguments.uri != null) {
     entryPoint = arguments.uri;
     memorySourceFiles = const <String, String>{};
   } else {
     entryPoint = Uri.parse('memory:main.dart');
     memorySourceFiles = SOURCE;
   }
 
   enableDebugMode();
 
   Compiler compiler1 = await compileWithDill(entryPoint, memorySourceFiles, [
     Flags.disableInlining,
     Flags.disableTypeInference
   ], beforeRun: (Compiler compiler) {
     compiler.backendStrategy = new JsBackendStrategy(compiler);
   }, printSteps: true);
   Expect.isFalse(compiler1.compilationFailed);
 }