Replace ClosedWorld.backend with ClosedWorld.backendClasses

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

Index: pkg/compiler/lib/src/native/enqueue.dart
diff --git a/pkg/compiler/lib/src/native/enqueue.dart b/pkg/compiler/lib/src/native/enqueue.dart
index 1ab06feb9c789a4834715a5f98a605230b813b59..8e2d48b1204226c52e54210e85ecc3eeafc05f50 100644
--- a/pkg/compiler/lib/src/native/enqueue.dart
+++ b/pkg/compiler/lib/src/native/enqueue.dart
@@ -483,8 +483,8 @@ abstract class NativeEnqueuerBase implements NativeEnqueuer {
           backend.registerInstantiatedType(type, world, registry);
         } else if (type == coreTypes.boolType) {
           backend.registerInstantiatedType(type, world, registry);
-        } else if (compiler.types
-            .isSubtype(type, backend.listImplementation.rawType)) {
+        } else if (compiler.types.isSubtype(
+            type, backend.backendClasses.listImplementation.rawType)) {
           backend.registerInstantiatedType(type, world, registry);
         }
         // TODO(johnniwinther): Improve spec string precision to handle type