Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/native_novel_html_test.dart

Index: tests/compiler/dart2js_native/native_novel_html_test.dart
diff --git a/tests/compiler/dart2js_native/native_novel_html_test.dart b/tests/compiler/dart2js_native/native_novel_html_test.dart
index e03589e77a19982d09d906741c298d0038f96033..01b5099b57ca1d7de788afd0a5deba8049c7038a 100644
--- a/tests/compiler/dart2js_native/native_novel_html_test.dart
+++ b/tests/compiler/dart2js_native/native_novel_html_test.dart
@@ -2,8 +2,7 @@
 // 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.
 
@@ -31,15 +30,11 @@ HTMLFakeyElement.prototype.nativeMethod = function(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();