[proxies] do not leak private symbols to proxy traps

test/mjsunit/harmony/proxies-set.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 --harmony-reflect
+// Flags: --harmony-proxies --harmony-reflect --allow-natives-syntax
 
 
 function sloppyDefaultSet(o, p, v) { return o[p] = v }
 function sloppyReflectSet(o, p, v) { return Reflect.set(o, p, v) }
 function strictDefaultSet(o, p, v) { "use strict"; return o[p] = v }
 function strictReflectSet(o, p, v) { "use strict"; return Reflect.set(o, p, v) }
 
 sloppyDefaultSet.shouldThrow = false;
 sloppyReflectSet.shouldThrow = false;
 strictDefaultSet.shouldThrow = true;
@@ skipping 287 matching lines @@
     for (var receiver of [null, undefined, 1]) {
       Reflect.set(object, p, 42, receiver);
       assertEquals(1, observations.length);
       assertArrayEquals([target, toKey(p), 42, receiver], observations[0]);
       assertSame(target, observations[0][0]);
       assertSame(receiver, observations[0][3]);
       observations = [];
     }
   }
 })();
+
+
+(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"));
+})();