Use JsStrategy, remove KernelBackendStrategy

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/kernel/kernel_backend_strategy.dart';
-import '../kernel/compile_from_dill_test_helper.dart';
-import '../serialization/helper.dart';
-
-main(List<String> args) {
-  useJsStrategyForTesting = true;
-  asyncTest(() async {
-    await mainInternal(args);
-  });
-}
-
-Future mainInternal(List<String> args,
-    {bool skipWarnings: false,
-    bool skipErrors: false,
-    List<String> options: const <String>[]}) async {
-  Arguments arguments = new Arguments.from(args);
-  List<Test> tests;
-  if (arguments.uri != null) {
-    tests = <Test>[new Test.fromUri(arguments.uri)];
-  } else {
-    tests = TESTS;
-  }
-  for (Test test in tests) {
-    if (test.uri != null) {
-      print('--- running test uri ${test.uri} -------------------------------');
-    } else {
-      print(
-          '--- running test code -------------------------------------------');
-      print(test.sources.values.first);
-      print('----------------------------------------------------------------');
-    }
-    await runTest(test.entryPoint, test.sources,
-        verbose: arguments.verbose,
-        skipWarnings: skipWarnings,
-        skipErrors: skipErrors,
-        options: options,
-        expectAstEquivalence: true,
-        expectIdenticalOutput: false);
-  }
-}