Revert "Add native_testing library to mock @Native classes"

tests/compiler/dart2js_native/uninstantiated_type_parameter_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 "dart:_js_helper";
+import "package:expect/expect.dart";
 
 // Test for uninstantiated native classes as type parameters.
 
 class UA {}
 
 @Native("B")
 class UB {}
 
 class C<T> {}
 
 main() {
   var a = new C<UA>();
   var b = new C<UB>();
 
   Expect.isTrue(a is! C<int>);
   Expect.isTrue(a is! C<C>);
 }