Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/event_loop_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 "dart:async";
+import "dart:_js_helper";
 import "package:async_helper/async_helper.dart";
-import "native_testing.dart";
+import "package:expect/expect.dart";
 
 typedef void Callback0();
 
 @Native("A")
 class A {
   foo(Callback0 f) native ;
 }
 
 makeA() native ;
 
 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
   // call from JS to Dart code.
   // We start by setting up a microtask that should only run after main has
   // finished. We then pass a closure into JavaScript. That closure is
   // immediately invoked. Dart2js had a bug, that it would start the event-loop
   // at this moment (a JS->Dart transition), and execute the scheduled
   // microtask.
 
   var events = [];
   asyncStart();
   var a = makeA();
   new Future.microtask(() {
     events.add("scheduleMicrotask");
   }).whenComplete(asyncEnd);
 
   Expect.equals(499, a.foo(() {
     events.add("closure to foo");
     return 499;
   }));
 
   events.add("after native call");
   Expect.listEquals(["closure to foo", "after native call"], events);
 }