Add test for making private symbols non-enumerable

test/mjsunit/regress/regress-private-enumerable.js

Index: test/mjsunit/regress/regress-private-enumerable.js
diff --git a/test/mjsunit/regress/regress-private-enumerable.js b/test/mjsunit/regress/regress-private-enumerable.js
new file mode 100644
index 0000000000000000000000000000000000000000..ad41b51baec6f0caf40ef35551edd8af214ceb59
--- /dev/null
+++ b/test/mjsunit/regress/regress-private-enumerable.js
@@ -0,0 +1,8 @@
+// Copyright 2016 the V8 project authors. 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 {}
+class B {}
+Object.assign(B, A);
+assertEquals("class B {}", B.toString());