Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/oddly_named_fields_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 'native_testing.dart';
 
 // JavaScript reserved words:
 //
 // break
 // case
 // catch
 // class
 // const
 // continue
 // debugger
@@ skipping 1361 matching lines @@
   if (!object.yieldValue) throw 'incorrect value in "yieldValue"';
   object.yieldValue = false;
   if (object.yieldValue) throw 'incorrect value in "yieldValue"';
 }
 
 NativeClassWithOddNames makeNativeClassWithOddNames() native ;
 
 setup() native """
 function NativeClassWithOddNames() {}
 makeNativeClassWithOddNames = function() { return new NativeClassWithOddNames; }
+self.nativeConstructor(NativeClassWithOddNames);
 """;
 
 main() {
+  nativeTesting();
   setup();
   var object = makeNativeClassWithOddNames();
   object.testMyFields();
   testObjectStronglyTyped(object);
   testObjectWeaklyTyped([object]);
   testObjectWeaklyTyped(['fisk']);
   testObjectWeaklyTyped([new ClassWithOddNames()..testMyFields()]);
 }