Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/js_constant_test.dart

Index: tests/compiler/dart2js_native/js_constant_test.dart
diff --git a/tests/compiler/dart2js_native/js_constant_test.dart b/tests/compiler/dart2js_native/js_constant_test.dart
index 7658dc07f467aac57ec6134b74199485219b1e1a..bc9837eb498710e6ea8170e6bd3589d8f1cac6a4 100644
--- a/tests/compiler/dart2js_native/js_constant_test.dart
+++ b/tests/compiler/dart2js_native/js_constant_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 'dart:_foreign_helper' show JS;
-import "package:expect/expect.dart";
+import 'native_testing.dart';
 
 // Negative constant numbers must be generated as negation, not just a literal
 // with a sign, i.e.