Mark native methods as external.

lib/custom_backend.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.md file.
 
 library rasta.custom_backend;
 
 import 'dart:async' show
     Future;
 
 import 'package:compiler/src/common/backend_api.dart' show
@@ skipping 108 matching lines @@
   @override bool enableDeferredLoadingIfSupported(
       Spannable node, Registry registry) => unimplemented;
 
   @override Uri resolvePatchUri(String libraryName, Uri plaformConfigUri) {
     return unimplemented;
   }
 
   @override MethodElement resolveExternalFunction(MethodElement element) {
     return patchResolver.resolveExternalFunction(element);
   }
+
+  @override bool canLibraryUseNative(LibraryElement library) {
+    return library.isPlatformLibrary || isNativeTest(library);
+  }
+
+  /// Returns true if [library] is a test of native keyword.
+  bool isNativeTest(LibraryElement library) {
+    Uri uri = library.canonicalUri;
+    return uri.scheme == "file" &&
+        uri.path.contains("/test/kernel/regression/native/");
+  }
 }