Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/hash_code_test.dart

 // Copyright (c) 2012, 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 "dart:_js_helper";
+import "package:expect/expect.dart";
 
 @Native("A")
 class A {}
 
-@Native("B")
-class B {
-  int get hashCode => 1234567;
-}
-
 makeA() native ;
-makeB() native ;
 
 void setup() native """
 function A() {}
-function B() {}
 makeA = function(){return new A;};
-makeB = function(){return new B;};
-
-self.nativeConstructor(A);
-self.nativeConstructor(B);
 """;
 
 main() {
-  nativeTesting();
   setup();
   Expect.isTrue(makeA().hashCode is int);
-  Expect.equals(1234567, makeB().hashCode);
 }