Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/load_elim_refinement_test.dart

 // Copyright (c) 2011, 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 'native_testing.dart';
+import "package:expect/expect.dart";
+import 'dart:_foreign_helper' show JS;
 
 class A {
   int a;
 }
 
 class B extends A {
   int b;
 }
 
 @NoInline()
 escape(v) { g = v; }
 var g;
 
 main() {
   g = new A();
   var a = JS('returns:A;new:true', '(1,#)', new B());
 
   a.a = 1;
   if (a is B) {
     escape(a);  // Here we need to escape 'a' not the refinement of a to B.
     g.a = 2;
     Expect.equals(2, a.a);
   }
 }