Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_novel_html_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';
 
 // Test to see if novel HTML tags are interpreted as HTMLElement.
 
 @Native("HTMLElement")
 class Element {
   String dartMethod(int x) => 'dartMethod(${nativeMethod(x+1)})';
   String nativeMethod(int x) native ;
 }
 
 makeE() native ;
 makeF() native ;
 
 void setup() native """
 // A novel HTML element.
 function HTMLGoofyElement(){}
 HTMLGoofyElement.prototype.nativeMethod = function(a) {
   return 'Goofy.nativeMethod(' + a  + ')';
 };
 makeE = function(){return new HTMLGoofyElement};
 
 // A non-HTML element with a misleading name.
 function HTMLFakeyElement(){}
 HTMLFakeyElement.prototype.nativeMethod = function(a) {
   return 'Fakey.nativeMethod(' + a  + ')';
 };
 makeF = function(){return new HTMLFakeyElement};
 
-// Make the HTMLGoofyElement look like a real host object.
-var theRealObjectToString = Object.prototype.toString;
-Object.prototype.toString = function() {
-  if (this instanceof HTMLGoofyElement) return '[object HTMLGoofyElement]';
-  return theRealObjectToString.call(this);
-}
+self.nativeConstructor(HTMLGoofyElement);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   var e = makeE();
   Expect.equals('Goofy.nativeMethod(10)', e.nativeMethod(10));
   Expect.equals('dartMethod(Goofy.nativeMethod(11))', e.dartMethod(10));
 
   var f = makeF();
   Expect.throws(() => f.nativeMethod(20), (e) => e is NoSuchMethodError,
       'fake HTML Element must not run Dart method on native class');
   Expect.throws(() => f.dartMethod(20), (e) => e is NoSuchMethodError,
       'fake HTML Element must not run native method on native class');
 }