Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/error_safeToString_test.dart

 // Copyright (c) 2015, 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_INTERCEPTOR_CONSTANT, JS;
-import 'dart:_js_helper' show Native, Creates;
+import "native_testing.dart";
+import 'dart:_foreign_helper' show JS_INTERCEPTOR_CONSTANT;
 import 'dart:_interceptors'
     show
         Interceptor,
         JavaScriptObject,
         PlainJavaScriptObject,
         UnknownJavaScriptObject;
 
 // Test for safe formatting of JavaScript objects by Error.safeToString.
 
 @Native('PPPP')
@@ skipping 45 matching lines @@
 
 function PPPP(){}
 makeP = function(){return new PPPP();};
 
 function QQQQ(){}
 makeQ = function(){return new QQQQ();};
 
 function RRRR(){}
 makeR = function(){return new RRRR();};
 
+self.nativeConstructor(PPPP);
+self.nativeConstructor(QQQQ);
+self.nativeConstructor(RRRR);
 """;
 
 expectTypeName(expectedName, s) {
   var m = new RegExp(r"Instance of '(.*)'").firstMatch(s);
   Expect.isNotNull(m);
   var name = m.group(1);
   Expect.isTrue(expectedName == name || name.length <= 3,
       "Is '$expectedName' or minified: '$name'");
 }
 
@@ skipping 68 matching lines @@
   x.toString();
   Expect.notEquals(plainJsString, Error.safeToString(x));
   Expect.notEquals(unknownJsString, Error.safeToString(x));
   Expect.notEquals(interceptorString, Error.safeToString(x));
   // And not the native class constructor.
   Expect.notEquals("Instance of 'RRRR'", Error.safeToString(x));
   expectTypeName('Rascal', Error.safeToString(x));
 }
 
 main() {
+  nativeTesting();
   setup();
 
   testDistinctInterceptors();
   testExternal();
   testNative();
 }