Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/subclassing_constructor_1_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 "package:expect/expect.dart";
-import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper' show Native, Creates, setNativeSubclassDispatchRecord;
+import "native_testing.dart";
+import 'dart:_js_helper' show setNativeSubclassDispatchRecord;
 import 'dart:_interceptors'
     show findInterceptorForType, findConstructorForNativeSubclassType;
 
 // Test that subclasses of native classes can be initialized by calling the
 // 'upgrade' constructor.
 
 var trace = [];
 
 var log;
 
@@ skipping 133 matching lines @@
     Expect.throws(action, (e) => true, "'$description must fail'");
   }
 
   checkThrows(() => new B.one(), 'new B.one()');
   checkThrows(() => new B.two(), 'new B.two()');
   checkThrows(() => new B.three(), 'new B.three()');
   checkThrows(() => new B.three(1), 'new B.three(1)');
   checkThrows(() => new B.three([]), 'new B.three([])');
 }
 
-var inscrutable;
-
 main() {
+  nativeTesting();
   setup();
-  inscrutable = (x) => x;
   log = (message) {
     trace.add('$message');
     return message;
   };
 
   setNativeSubclassDispatchRecord(getBPrototype(), findInterceptorForType(B));
 
   test_one();
   test_two();
   test_three();
   test_new();
 }