[proxies] do not leak private symbols to proxy traps

test/mjsunit/harmony/proxies-set.js

Index: test/mjsunit/harmony/proxies-set.js
diff --git a/test/mjsunit/harmony/proxies-set.js b/test/mjsunit/harmony/proxies-set.js
index 2fec115a103ea05d0a49fa350381e157f679487a..1b306f8c6ed57cbc2a9a0b42d748d4efd33372f5 100644
--- a/test/mjsunit/harmony/proxies-set.js
+++ b/test/mjsunit/harmony/proxies-set.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
 
 
 function sloppyDefaultSet(o, p, v) { return o[p] = v }
@@ -310,3 +310,16 @@ TestTrapReceiverArgument(strictReflectSet);
     }
   }
 })();
+
+
+(function testSetPrivateSymbol() {
+  var symbol = %CreatePrivateSymbol("secret");
+  var O = {};
+  var proxy = new Proxy(O, { set(t, name, val) { assertUnreachable(); }});
+  O[symbol] = "value";
+  proxy[symbol] = "value";
+  assertEquals(undefined, proxy[symbol]);
+  assertEquals(false, Reflect.set(proxy, symbol, "otherValue"));
+  assertEquals("value", O[symbol]);
+  assertEquals(true, Reflect.set(O, symbol, "otherValue"));
+})();