Assert that we don't mix K and J elements

tests/compiler/dart2js/kernel/compile_from_dill_test_helper.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.
 
 // Helper to test compilation equivalence between source and .dill based
 // compilation.
 library dart2js.kernel.compile_from_dill_test_helper;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:compiler/compiler_new.dart';
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/common.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/types.dart';
 import 'package:compiler/src/kernel/element_map.dart';
+import 'package:compiler/src/kernel/kernel_backend_strategy.dart';
 import 'package:compiler/src/kernel/kernel_strategy.dart';
 import 'package:compiler/src/serialization/equivalence.dart';
 import 'package:compiler/src/resolution/enum_creator.dart';
 import 'package:compiler/src/universe/world_builder.dart';
 import 'package:compiler/src/world.dart';
 import 'package:expect/expect.dart';
 import '../memory_compiler.dart';
 import '../equivalence/check_functions.dart';
 import '../equivalence/check_helpers.dart';
 import '../serialization/helper.dart';
@@ skipping 200 matching lines @@
   OutputCollector collector2 = new OutputCollector();
   Compiler compiler2 = await compileWithDill(
       entryPoint, const {}, <String>[]..addAll(commonOptions)..addAll(options),
       printSteps: true, compilerOutput: collector2);
 
   KernelFrontEndStrategy frontendStrategy = compiler2.frontendStrategy;
   KernelToElementMap elementMap = frontendStrategy.elementMap;
 
   Expect.isFalse(compiler2.compilationFailed);
 
-  KernelEquivalence equivalence = new KernelEquivalence(elementMap);
+  KernelEquivalence equivalence1 = new KernelEquivalence(elementMap);
 
   ClosedWorld closedWorld2 =
       compiler2.resolutionWorldBuilder.closedWorldForTesting;
 
   checkBackendUsage(closedWorld1.backendUsage, closedWorld2.backendUsage,
-      equivalence.defaultStrategy);
+      equivalence1.defaultStrategy);
 
   print('--- checking resolution enqueuers ----------------------------------');
   checkResolutionEnqueuers(closedWorld1.backendUsage, closedWorld2.backendUsage,
       compiler1.enqueuer.resolution, compiler2.enqueuer.resolution,
-      elementEquivalence: (a, b) => equivalence.entityEquivalence(a, b),
+      elementEquivalence: (a, b) => equivalence1.entityEquivalence(a, b),
       typeEquivalence: (DartType a, DartType b) {
-        return equivalence.typeEquivalence(unalias(a), b);
+        return equivalence1.typeEquivalence(unalias(a), b);
       },
       elementFilter: elementFilter,
       verbose: verbose);
 
   print('--- checking closed worlds -----------------------------------------');
   checkClosedWorlds(closedWorld1, closedWorld2,
-      strategy: equivalence.defaultStrategy,
+      strategy: equivalence1.defaultStrategy,
       verbose: verbose,
       // TODO(johnniwinther,efortuna): Require closure class equivalence when
       // these are supported.
       allowMissingClosureClasses: true);
 
   // TODO(johnniwinther): Perform equivalence tests on the model: codegen world
   // impacts, program model, etc.
 
   print('--- checking codegen enqueuers--------------------------------------');
+
+  KernelBackendStrategy backendStrategy = compiler2.backendStrategy;
+  KernelEquivalence equivalence2 =
+      new KernelEquivalence(backendStrategy.elementMap);
+
   checkCodegenEnqueuers(compiler1.enqueuer.codegenEnqueuerForTesting,
       compiler2.enqueuer.codegenEnqueuerForTesting,
-      elementEquivalence: (a, b) => equivalence.entityEquivalence(a, b),
+      elementEquivalence: (a, b) => equivalence2.entityEquivalence(a, b),
       typeEquivalence: (DartType a, DartType b) {
-        return equivalence.typeEquivalence(unalias(a), b);
+        return equivalence2.typeEquivalence(unalias(a), b);
       },
       elementFilter: elementFilter,
       verbose: verbose);
 
   checkEmitters(compiler1.backend.emitter, compiler2.backend.emitter,
-      elementEquivalence: (a, b) => equivalence.entityEquivalence(a, b),
+      elementEquivalence: (a, b) => equivalence2.entityEquivalence(a, b),
       typeEquivalence: (DartType a, DartType b) {
-        return equivalence.typeEquivalence(unalias(a), b);
+        return equivalence2.typeEquivalence(unalias(a), b);
       },
       verbose: verbose);
 
   if (expectAstEquivalence) {
     checkGeneratedCode(compiler1.backend, compiler2.backend,
-        elementEquivalence: (a, b) => equivalence.entityEquivalence(a, b));
+        elementEquivalence: (a, b) => equivalence2.entityEquivalence(a, b));
   }
 
   if (expectIdenticalOutput) {
     print('--- checking output------- ---------------------------------------');
     collector1.outputMap
         .forEach((OutputType outputType, Map<String, BufferedOutputSink> map1) {
       if (outputType == OutputType.sourceMap) {
         // TODO(johnniwinther): Support source map from .dill.
         return;
       }
       Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
       checkSets(map1.keys, map2.keys, 'output', equality);
       map1.forEach((String name, BufferedOutputSink output1) {
         BufferedOutputSink output2 = map2[name];
         Expect.stringEquals(output1.text, output2.text);
       });
     });
   }
   return ResultKind.success;
 }