Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart

Index: tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart b/tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart
index 3f7544aaad853b2b2856c3d34063cb614260eed8..3ba8cc8031bd0b207598f8ed8094c0e7bcb9875b 100644
--- a/tests/compiler/dart2js_native/native_class_inheritance4_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_class_inheritance4_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";
 
 // Additional Dart code may be 'placed on' hidden native classes.  With
 // inheritance, the superclass method must not be reached by a call on the
@@ -53,6 +52,9 @@ function B(){}
 inherits(B, A);
 makeA = function(){return new A;};
 makeB = function(){return new B;};
+
+self.nativeConstructor(A);
+self.nativeConstructor(B);
 """;
 
 testBasicA_dynamic() {
@@ -145,6 +147,7 @@ testAB_typed() {
 }
 
 main() {
+  nativeTesting();
   testBasicA_dynamic();
   testBasicA_typed();
   testBasicB_dynamic();