[proxies] do not leak private symbols to proxy traps

test/mjsunit/harmony/proxies-delete-property.js

Index: test/mjsunit/harmony/proxies-delete-property.js
diff --git a/test/mjsunit/harmony/proxies-delete-property.js b/test/mjsunit/harmony/proxies-delete-property.js
index 27f9c059ccd6815cf4481d11ae33fe563ba3ec0c..f69f25535ca29270f648a8f95c7c529d125e1b8e 100644
--- a/test/mjsunit/harmony/proxies-delete-property.js
+++ b/test/mjsunit/harmony/proxies-delete-property.js
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-proxies --harmony-reflect
+// Flags: --harmony-proxies --harmony-reflect --allow-natives-syntax
 
 
 var properties =
@@ -188,3 +188,14 @@ TestTrappingFalsish(
     (o, p) => {"use strict"; return delete o[p]}, true);
 TestTrappingFalsish(
     (o, p) => {"use strict"; return Reflect.deleteProperty(o, p)}, false);
+
+(function testDeletePrivateSymbol() {
+  var symbol = %CreatePrivateSymbol("secret");
+  var O = { [symbol]: "value" };
+  var proxy = new Proxy(O, { delete(t, name) { assertUnreachable(); }});
+  delete proxy[symbol];
+  assertEquals(true, Reflect.deleteProperty(proxy, symbol));
+  assertEquals("value", O[symbol]);
+  assertEquals(true, Reflect.deleteProperty(O, symbol));
+  assertEquals(undefined, O[symbol]);
+})();