ES6 symbols: symbol properties should not cause going into slow mode.

test/mjsunit/harmony/symbols.js

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 195 matching lines @@
     assertTrue(weakmap.delete(symbols[i]))
   }
   assertEquals(0, set.size)
   assertEquals(0, map.size)
 }
 TestCollections()
 
 
 
 function TestKeySet(obj) {
+  assertTrue(%HasFastProperties(obj))
   // Set the even symbols via assignment.
   for (var i = 0; i < symbols.length; i += 2) {
     obj[symbols[i]] = i
+    // Object should remain in fast mode until too many properties were added.
+    assertTrue(%HasFastProperties(obj) || i >= 30)
   }
 }
 
 
 function TestKeyDefine(obj) {
   // Set the odd symbols via defineProperty (as non-enumerable).
   for (var i = 1; i < symbols.length; i += 2) {
     Object.defineProperty(obj, symbols[i], {value: i, configurable: true})
   }
 }
@@ skipping 62 matching lines @@
   var obj = objs[i]
   TestKeySet(obj)
   TestKeyDefine(obj)
   TestKeyGet(obj)
   TestKeyHas(obj)
   TestKeyEnum(obj)
   TestKeyNames(obj)
   TestKeyDescriptor(obj)
   TestKeyDelete(obj)
 }