Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/native_to_string_frog_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 {
-  toString() => 'AAA';
-}
+class A {}
 
 makeA() native ;
 
 void setup() native """
 function A() {}
 makeA = function(){return new A;};
-self.nativeConstructor(A);
 """;
 
 main() {
-  nativeTesting();
   setup();
-
   Expect.isTrue(makeA().toString() is String);
-  Expect.equals('AAA', makeA().toString());
-
-  Expect.isTrue(confuse(makeA()).toString() is String);
-  Expect.equals('AAA', confuse(makeA()).toString());
 }