[proxies] do not leak private symbols to proxy traps

test/mjsunit/harmony/proxies-has.js

 // Copyright 2015 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.
 
-// Flags: --harmony-proxies
+// Flags: --harmony-proxies --harmony-reflect --allow-natives-syntax
 
 var target = {
   "target_one": 1
 };
 target.__proto__ = {
   "target_two": 2
 };
 var handler = {
   has: function(target, name) {
     return name == "present";
@@ skipping 31 matching lines @@
 result = false;
 assertThrows("'nonconf' in proxy", TypeError);
 
 // Step 9b iii. Trap result must confirm presence of all own properties of
 // non-extensible targets.
 Object.freeze(target);
 assertThrows("'nonconf' in proxy", TypeError);
 assertThrows("'target_one' in proxy", TypeError);
 assertFalse("target_two" in proxy);
 assertFalse("in_your_dreams" in proxy);
+
+(function testHasPrivateSymbol() {
+  var symbol = %CreatePrivateSymbol("secret");
+  var O = {};
+  var proxy = new Proxy(O, { has(t, name) { assertUnreachable(); }});
+  O[symbol] = "value";
+  proxy[symbol] = "value";
+  assertEquals(false, symbol in proxy);
+  assertEquals(false, Reflect.has(proxy, symbol));
+  assertEquals(true, symbol in O);
+  assertEquals(true, Reflect.has(O, symbol));
+})();