Assert that we don't mix K and J elements

pkg/compiler/lib/src/native/resolver.dart

 // Copyright (c) 2014, 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 'package:front_end/src/fasta/scanner.dart' show StringToken, Token;
 import 'package:front_end/src/fasta/scanner.dart' as Tokens show EOF_TOKEN;
 import 'package:front_end/src/scanner/token.dart' show BeginToken;
 
 import '../common.dart';
 import '../common_elements.dart' show CommonElements, ElementEnvironment;
@@ skipping 202 matching lines @@
     implements NativeDataResolver {
   final Compiler _compiler;
 
   NativeDataResolverImpl(this._compiler);
 
   JavaScriptBackend get _backend => _compiler.backend;
   DiagnosticReporter get _reporter => _compiler.reporter;
   ElementEnvironment get elementEnvironment =>
       _compiler.resolution.elementEnvironment;
   CommonElements get commonElements => _compiler.resolution.commonElements;
-  NativeBasicData get nativeBasicData => _backend.nativeBasicData;
+  NativeBasicData get nativeBasicData =>
+      _compiler.frontendStrategy.nativeBasicData;
   NativeDataBuilder get nativeDataBuilder => _backend.nativeDataBuilder;
 
   @override
   bool isJsInteropMember(MemberElement element) {
     // TODO(johnniwinther): Avoid computing this twice for external function;
     // once from JavaScriptBackendTarget.resolveExternalFunction and once
     // through JavaScriptBackendTarget.resolveNativeMember.
     bool isJsInterop =
         checkJsInteropMemberAnnotations(_compiler, element, nativeDataBuilder);
     // TODO(johnniwinther): Avoid this duplication of logic from
@@ skipping 384 matching lines @@
 
   Iterable<ConstantValue> fields = constructedObject.fields.values;
   // TODO(sra): Better validation of the constant.
   if (fields.length != 1 || fields.single is! StringConstantValue) {
     throw new SpannableAssertionFailure(
         spannable, 'Annotations needs one string: ${value.toStructuredText()}');
   }
   StringConstantValue specStringConstant = fields.single;
   return specStringConstant.primitiveValue;
 }