Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/fake_thing_2_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 that native objects cannot accidentally or maliciously be mistaken for
 // Dart objects.
 // The difference between fake_thing_test and fake_thing_2_test is the
 // presence of a used declared native class.
 
 class Thing {}
 
 @Native("NT")
 class NativeThing {}
 
 make1() native ;
 make2() native ;
 make3() native ;
 
 void setup() native r"""
 function A() {}
 A.prototype.$isThing = true;
 make1 = function(){return new A;};
 make2 = function(){return {$isThing: true}};
 function NT() {}
 NT.prototype.$isThing = true;
 make3 = function(){return new NT;};
+
+self.nativeConstructor(NT);
 """;
 
-var inscrutable;
 main() {
+  nativeTesting();
   setup();
-  inscrutable = (x) => x;
 
   var a = new Thing();
   var b = make1();
   var c = make2();
   var d = make3();
-  Expect.isTrue(inscrutable(a) is Thing);
-  Expect.isFalse(inscrutable(b) is Thing);
-  Expect.isFalse(inscrutable(c) is Thing);
-  Expect.isFalse(inscrutable(d) is Thing);
+  Expect.isTrue(confuse(a) is Thing);
+  Expect.isFalse(confuse(b) is Thing);
+  Expect.isFalse(confuse(c) is Thing);
+  Expect.isFalse(confuse(d) is Thing);
 }