Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/fixup_get_tag_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 @Native("A")
 class Foo {
   // There is one native class with dispatch tag 'A'.
   token() native ;
 }
 
 void setup() native r"""
@@ skipping 10 matching lines @@
 };
 
 function A(){ }
 A.prototype.token = function () { return 'isA'; }
 
 function B(){ }
 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();
   var b = makeB();
 
   Expect.equals('isA', a.token());
 
   // This call succeeds because the fixed-up 'getTag' method returns Foo's
   // dispatch tag, and B is a faithful polyfil for Foo/A.
   Expect.equals('isB', b.token());
 
   Expect.isTrue(a is Foo);
   Expect.isTrue(b is Foo);
 }