Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_checked_fields_frog_test.dart

Index: tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart b/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
index 3ae79bf903f58163083eae4c9921a1f04f673cb0..83f5bac1c904199e13751e59cc43283c9785a4b5 100644
--- a/tests/compiler/dart2js_native/native_checked_fields_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_checked_fields_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 that type checks occur on assignment to fields of native methods.
 
@@ -27,6 +26,9 @@ function B() {}
 
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 expectThrows(action()) {
@@ -109,6 +111,7 @@ bool isCheckedMode() {
 }
 
 main() {
+  nativeTesting();
   setup();
 
   if (isCheckedMode()) {