Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_closure_identity_frog_test.dart

Index: tests/compiler/dart2js_native/native_closure_identity_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_closure_identity_frog_test.dart b/tests/compiler/dart2js_native/native_closure_identity_frog_test.dart
index 2a27d5dce8015334761dc6cb616c07296e224d63..3d73c35617a58236088e7d5fde5e1c76d5fc61a9 100644
--- a/tests/compiler/dart2js_native/native_closure_identity_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_closure_identity_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';
 
 typedef void MyFunctionType();
 
@@ -22,6 +21,8 @@ A.prototype.setClosure = function(f) { this.f = f; };
 A.prototype.check = function(f) { return this.f === f; };
 A.prototype.invoke = function() { return this.f(); };
 makeA = function(){return new A;};
+
+self.nativeConstructor(A);
 """;
 
 var staticClosure;
@@ -45,6 +46,7 @@ checkTyped(A a, MyFunctionType closure) {
 }
 
 main() {
+  nativeTesting();
   setup();
 
   staticClosure = () => 44;