Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_rename_2_frog_test.dart

 // Copyright (c) 2011, 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.
 
 // A native method prevents other members from having that name, including
 // fields.  However, native fields keep their name.  The implication: a getter
 // for the field must be based on the field's name, not the field's jsname.
 
-import "package:expect/expect.dart";
-import 'dart:_js_helper' show Native, JSName;
+import 'native_testing.dart';
 
 abstract class I {
   int key;
 }
 
 @Native("A")
 class A implements I {
   int key; //  jsname is 'key'
   int getKey() => key;
 }
@@ skipping 19 matching lines @@
 void setup() native """
 // This code is all inside 'setup' and so not accesible from the global scope.
 function A(){ this.key = 111; }
 A.prototype.getKey = function(){return this.key;};
 
 function X(){}
 X.prototype.key = function(){return 666;};
 
 makeA = function(){return new A};
 makeX = function(){return new X};
+
+self.nativeConstructor(A);
+self.nativeConstructor(X);
 """;
 
 testDynamic() {
-  var things = [makeA(), new B(), makeX()];
-  var a = things[0];
-  var b = things[1];
-  var x = things[2];
+  var a = confuse(makeA());
+  var b = confuse(new B());
+  var x = confuse(makeX());
 
   Expect.equals(111, a.key);
   Expect.equals(222, b.key);
   Expect.equals(111, a.getKey());
   Expect.equals(222, b.getKey());
 
   Expect.equals(666, x.native_key_method());
   Expect.equals(666, x.key());
   var fn = x.key;
   Expect.equals(666, fn());
@@ skipping 17 matching lines @@
   X x = makeX();
 
   Expect.equals(666, x.native_key_method());
   Expect.equals(111, a.key);
   Expect.equals(222, b.key);
   Expect.equals(111, a.getKey());
   Expect.equals(222, b.getKey());
 }
 
 main() {
+  nativeTesting();
   setup();
 
   testTyped();
   testPartial();
   testDynamic();
 }