Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_field_name_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.
 
-import "dart:_js_helper";
-import "package:expect/expect.dart";
+import 'native_testing.dart';
 
 // Check that native fields are not incorrectly renamed.
 
 @Native("A")
 class A {
   int myLongPropertyName;
   int getValue;
 
   int method(int z) => myLongPropertyName;
 }
 
 // This code is inside the setup function, so the function names are not
 // accessible, but the makeA variable is global through the magic of JS scoping.
 // The contents of this are of course not analyzable by the compiler.
 void setup() native r"""
 function getter() {
   return ++this.getValue;
 }
 
 function setter(x) {
   this.getValue += 10;
 }
 
 function A(){
   var a = Object.create(
-      { constructor: { name: 'A'}},
+      { constructor: A },
       { myLongPropertyName: { get: getter,
                               set: setter,
                               configurable: false,
                               writeable: false
                             }
       });
   a.getValue = 0;
   return a;
 }
 
 makeA = function(){return new A;};
+
+self.nativeConstructor(A);
 """;
 
-A makeA() native ;
+/*A*/ makeA() native ;
 
 main() {
+  nativeTesting();
   setup();
   var a = makeA();
   a.myLongPropertyName = 21;
   int gotten = a.myLongPropertyName;
   Expect.equals(11, gotten);
 
   var a2 = makeA();
   if (a2 is A) {
     // Inside this 'if' the compiler knows that a2 is an A, so it is tempted
     // to access myLongPropertyName directly, using its minified name.  But
     // renaming of native properties can only work using getters and setters
     // that access the original name.
     a2.myLongPropertyName = 21;
     int gotten = a2.myLongPropertyName;
     Expect.equals(11, gotten);
   }
 }