Assert that we don't mix K and J elements

tests/compiler/dart2js/serialization/native_data_test.dart

 // Copyright (c) 2016, 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.
 
 library dart2js.serialization.native_data_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/common/names.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart';
-import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/js_backend/native_data.dart';
 import 'package:compiler/src/filenames.dart';
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/world.dart';
 import '../equivalence/check_helpers.dart';
 import '../memory_compiler.dart';
 import 'helper.dart';
 
 main(List<String> args) {
   asyncTest(() async {
@@ skipping 22 matching lines @@
   print('------------------------------------------------------------------');
   Compiler compiler2 = compilerFor(
       memorySourceFiles: serializedData.toMemorySourceFiles(),
       resolutionInputs: serializedData.toUris(),
       options: [Flags.analyzeAll]);
   await compiler2.run(uri);
   var elementEnvironment2 = compiler2.frontendStrategy.elementEnvironment;
   ClosedWorld closedWorld2 =
       compiler2.closeResolution(elementEnvironment2.mainFunction).closedWorld;
 
-  JavaScriptBackend backend1 = compiler1.backend;
-  JavaScriptBackend backend2 = compiler2.backend;
-  NativeBasicDataImpl nativeBasicData1 = backend1.nativeBasicData;
-  NativeBasicDataImpl nativeBasicData2 = backend2.nativeBasicData;
+  NativeBasicDataImpl nativeBasicData1 =
+      compiler1.frontendStrategy.nativeBasicData;
+  NativeBasicDataImpl nativeBasicData2 =
+      compiler2.frontendStrategy.nativeBasicData;
   NativeDataImpl nativeData1 = closedWorld1.nativeData;
   NativeDataImpl nativeData2 = closedWorld2.nativeData;
 
   checkMaps(
       nativeData1.jsInteropLibraryNames,
       nativeData2.jsInteropLibraryNames,
       "NativeData.jsInteropLibraryNames",
       areElementsEquivalent,
       equality,
       verbose: verbose);
@@ skipping 35 matching lines @@
       verbose: verbose);
 
   checkMaps(
       nativeData1.nativeFieldStoreBehavior,
       nativeData2.nativeFieldStoreBehavior,
       "NativeData.nativeFieldStoreBehavior",
       areElementsEquivalent,
       testNativeBehavior,
       verbose: verbose);
 }