Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/error_safeToString_test.dart

Index: tests/compiler/dart2js_native/error_safeToString_test.dart
diff --git a/tests/compiler/dart2js_native/error_safeToString_test.dart b/tests/compiler/dart2js_native/error_safeToString_test.dart
index fa022d9c94a34213cdb1fe2e40bfa5fe13ef5a84..7e117f9afaaf7f1d52bf110f9bd5c470f3bd291d 100644
--- a/tests/compiler/dart2js_native/error_safeToString_test.dart
+++ b/tests/compiler/dart2js_native/error_safeToString_test.dart
@@ -2,9 +2,8 @@
 // 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,
@@ -70,6 +69,9 @@ makeQ = function(){return new QQQQ();};
 function RRRR(){}
 makeR = function(){return new RRRR();};
 
+self.nativeConstructor(PPPP);
+self.nativeConstructor(QQQQ);
+self.nativeConstructor(RRRR);
 """;
 
 expectTypeName(expectedName, s) {
@@ -158,6 +160,7 @@ testNative() {
 }
 
 main() {
+  nativeTesting();
   setup();
 
   testDistinctInterceptors();