Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/fake_thing_2_test.dart

Index: tests/compiler/dart2js_native/fake_thing_2_test.dart
diff --git a/tests/compiler/dart2js_native/fake_thing_2_test.dart b/tests/compiler/dart2js_native/fake_thing_2_test.dart
index 3ec92e022e213b9838dfea8e9a9785bdf3035df9..157e08f5d2d55fdf0cf8d7628c8cb259259fb8b1 100644
--- a/tests/compiler/dart2js_native/fake_thing_2_test.dart
+++ b/tests/compiler/dart2js_native/fake_thing_2_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:_js_helper";
-import "package:expect/expect.dart";
+import 'native_testing.dart';
 
 // Test that native objects cannot accidentally or maliciously be mistaken for
 // Dart objects.
@@ -27,19 +26,20 @@ make2 = function(){return {$isThing: true}};
 function NT() {}
 NT.prototype.$isThing = true;
 make3 = function(){return new NT;};
+
+self.nativeConstructor(NT);
 """;
 
-var inscrutable;
 main() {
+  nativeTesting();
   setup();
-  inscrutable = (x) => x;
 
   var a = new Thing();
   var b = make1();
   var c = make2();
   var d = make3();
-  Expect.isTrue(inscrutable(a) is Thing);
-  Expect.isFalse(inscrutable(b) is Thing);
-  Expect.isFalse(inscrutable(c) is Thing);
-  Expect.isFalse(inscrutable(d) is Thing);
+  Expect.isTrue(confuse(a) is Thing);
+  Expect.isFalse(confuse(b) is Thing);
+  Expect.isFalse(confuse(c) is Thing);
+  Expect.isFalse(confuse(d) is Thing);
 }