Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/load_elim_refinement_test.dart

Index: tests/compiler/dart2js_native/load_elim_refinement_test.dart
diff --git a/tests/compiler/dart2js_native/load_elim_refinement_test.dart b/tests/compiler/dart2js_native/load_elim_refinement_test.dart
index 026b142ac3eab8d90a4118a75522d501466d7a1f..981c0fa19019ef20999b0690b272ae81ee033889 100644
--- a/tests/compiler/dart2js_native/load_elim_refinement_test.dart
+++ b/tests/compiler/dart2js_native/load_elim_refinement_test.dart
@@ -2,8 +2,7 @@
 // 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:expect/expect.dart";
-import 'dart:_foreign_helper' show JS;
+import 'native_testing.dart';
 
 class A {
   int a;