Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_class_inheritance3_frog_test.dart

Index: tests/compiler/dart2js_native/native_class_inheritance3_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_class_inheritance3_frog_test.dart b/tests/compiler/dart2js_native/native_class_inheritance3_frog_test.dart
index 9c23566b8d2defdb3f942243e391cbb87c7f3cf3..900b011af624afddaede08e2cd4abc00dfa27d14 100644
--- a/tests/compiler/dart2js_native/native_class_inheritance3_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_class_inheritance3_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 to see if resolving a hidden native class's method to noSuchMethod
 // interferes with subsequent resolving of the method.  This might happen if the
@@ -59,9 +58,15 @@ makeA2 = function(){return new A2};
 makeB2 = function(){return new B2};
 
 makeObject = function(){return new Object};
+
+self.nativeConstructor(A1);
+self.nativeConstructor(A2);
+self.nativeConstructor(B1);
+self.nativeConstructor(B2);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   var a1 = makeA1();