Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/fixup_get_tag_test.dart

Index: tests/compiler/dart2js_native/fixup_get_tag_test.dart
diff --git a/tests/compiler/dart2js_native/fixup_get_tag_test.dart b/tests/compiler/dart2js_native/fixup_get_tag_test.dart
index 8fc9b70cdc6525144cb331b42fa51fea9120398d..160e41a78778847c0df257b07d42b15f697c0752 100644
--- a/tests/compiler/dart2js_native/fixup_get_tag_test.dart
+++ b/tests/compiler/dart2js_native/fixup_get_tag_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 for dartExperimentalFixupGetTag.
 
@@ -34,12 +33,15 @@ B.prototype.token = function () { return 'isB'; }
 
 makeA = function() { return new A; };
 makeB = function() { return new B; };
+
+self.nativeConstructor(A);
 """;
 
 makeA() native ;
 makeB() native ;
 
 main() {
+  nativeTesting();
   setup();
 
   var a = makeA();