Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_exceptions1_frog_test.dart

Index: tests/compiler/dart2js_native/native_exceptions1_frog_test.dart
diff --git a/tests/compiler/dart2js_native/native_exceptions1_frog_test.dart b/tests/compiler/dart2js_native/native_exceptions1_frog_test.dart
index a1116502fc8e840f9bb31b234313cfe531a5b414..9ab42836af440b2e3acc064df1d30cd346e7a155 100644
--- a/tests/compiler/dart2js_native/native_exceptions1_frog_test.dart
+++ b/tests/compiler/dart2js_native/native_exceptions1_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 hidden native exception classes can be marked as existing.
 //
@@ -57,11 +56,15 @@ A.prototype.op = function (x) {
   return  x / 2;
 };
 makeA = function(){return new A};
+
+self.nativeConstructor(E);
+self.nativeConstructor(A);
 """;
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 main() {
+  nativeTesting();
   setup1();
   setup2();