Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart

Index: tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart b/tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart
index e6d55fe0a67ceafee48a441427506f025aa5410e..f24c3552f628911a69311e00ab92975f0ed7ff82 100644
--- a/tests/compiler/dart2js_native/native_class_is_check3_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_class_is_check3_frog_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 for correct simple is-checks on hidden native classes.
 
@@ -48,11 +47,14 @@ A.prototype.read = function() { return this._x; };
 A.prototype.write = function(x) { this._x = x; };
 makeA = function(){return new A};
 makeB = function(){return new B};
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 class C {}
 
 main() {
+  nativeTesting();
   setup();
 
   var a1 = makeA();