Assert that we don't mix K and J elements

pkg/compiler/lib/src/js_backend/backend_serialization.dart

Index: pkg/compiler/lib/src/js_backend/backend_serialization.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_serialization.dart b/pkg/compiler/lib/src/js_backend/backend_serialization.dart
index 907d61781f03e5d3fd2d9cb3ec3596e23caf7102..b28bdc17a19d058ae66ba3c27215c421673b2da9 100644
--- a/pkg/compiler/lib/src/js_backend/backend_serialization.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_serialization.dart
@@ -10,6 +10,7 @@ import '../elements/resolution_types.dart';
 import '../elements/types.dart';
 import '../js/js.dart' as js;
 import '../native/native.dart';
+import '../resolution/resolution_strategy.dart';
 import '../serialization/keys.dart';
 import '../serialization/serialization.dart'
     show DeserializerPlugin, ObjectDecoder, ObjectEncoder, SerializerPlugin;
@@ -53,7 +54,8 @@ class JavaScriptBackendSerializer implements SerializerPlugin {
 
   JavaScriptBackendSerializer(this._backend);
 
-  NativeBasicDataImpl get nativeBasicData => _backend.nativeBasicData;
+  NativeBasicDataImpl get nativeBasicData =>
+      _backend.compiler.frontendStrategy.nativeBasicData;
   NativeDataBuilderImpl get nativeData => _backend.nativeDataBuilder;
 
   @override
@@ -117,8 +119,12 @@ class JavaScriptBackendDeserializer implements DeserializerPlugin {
 
   JavaScriptBackendDeserializer(this._backend);
 
-  NativeBasicDataBuilderImpl get nativeBasicData =>
-      _backend.nativeBasicDataBuilder;
+  NativeBasicDataBuilderImpl get nativeBasicData {
+    ResolutionFrontEndStrategy frontendStrategy =
+        _backend.compiler.frontendStrategy;
+    return frontendStrategy.nativeBasicDataBuilder;
+  }
+
   NativeDataBuilderImpl get nativeData => _backend.nativeDataBuilder;
 
   @override