Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/foreign_test.dart

Index: tests/compiler/dart2js_native/foreign_test.dart
diff --git a/tests/compiler/dart2js_native/foreign_test.dart b/tests/compiler/dart2js_native/foreign_test.dart
index e2a4c4e847f28fb6f907eef0bdcc9f4106f62bf8..c1d1c65b4f54e6e8b8a991264b69d2c028861bf2 100644
--- a/tests/compiler/dart2js_native/foreign_test.dart
+++ b/tests/compiler/dart2js_native/foreign_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";
 
 foreign1(var a, var b) {
   return JS("num", r"# + #", a, b);