Move frog/tests/{leg,leg_only,frog_native} to tests/compiler/.

frog/tests/frog_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.
-
-class A native "*A" {}
-makeA() native;
-
-void setup() native """
-function A() {}
-makeA = function(){return new A;};
-""";
-
-
-main() {
-  setup();
-  Expect.isTrue(makeA().toString() is String);
-}