Add native_testing library to mock @Native classes

tests/compiler/dart2js_native/event_loop_test.dart

Index: tests/compiler/dart2js_native/event_loop_test.dart
diff --git a/tests/compiler/dart2js_native/event_loop_test.dart b/tests/compiler/dart2js_native/event_loop_test.dart
index 18d206891d52046850005d1b435acfeca6816ff9..60dd77a57f685b31135cc936ca26ad1446aa8c30 100644
--- a/tests/compiler/dart2js_native/event_loop_test.dart
+++ b/tests/compiler/dart2js_native/event_loop_test.dart
@@ -3,9 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import "dart:async";
-import "dart:_js_helper";
 import "package:async_helper/async_helper.dart";
-import "package:expect/expect.dart";
+import "native_testing.dart";
 
 typedef void Callback0();
 
@@ -20,9 +19,11 @@ void setup() native r"""
 function A() {}
 A.prototype.foo = function(f) { return f(); };
 makeA = function() { return new A; };
+self.nativeConstructor(A);
 """;
 
 main() {
+  nativeTesting();
   setup();
 
   // Makes sure that we don't run the event-loop when we have a reentrant