Mark native methods as external.

test/kernel/regression/native_is_illegal.dart

Index: test/kernel/regression/native_is_illegal.dart
diff --git a/test/kernel/regression/abstract_constructor.dart b/test/kernel/regression/native_is_illegal.dart
similarity index 57%
copy from test/kernel/regression/abstract_constructor.dart
copy to test/kernel/regression/native_is_illegal.dart
index 8caa4edb851dff34558b9bbd83d76f4a68bbc54d..84b8275ca93199edcf828de6704b5a6cc0749b80 100644
--- a/test/kernel/regression/abstract_constructor.dart
+++ b/test/kernel/regression/native_is_illegal.dart
@@ -2,8 +2,11 @@
 // 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.
 
-abstract class C {}
+foo() native "foo";
 
-main() {
-  new C();
+class Bar {
+  Bar get x native "Bar_get_x";
+  set x(Bar value) native "Bar_set_x";
+  f() native "Bar_f";
+  factory Bar() native "Bar_constructor";
 }