Don't turn objects with empty-string properties into fast-mode.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/165743003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19511 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/objects.cc

@@ -15710,6 +15710,7 @@ MaybeObject* NameDictionary::TransformPropertiesToFastFor(
         // instance descriptor.
         MaybeObject* maybe_key = heap->InternalizeString(String::cast(k));
         if (!maybe_key->To(&key)) return maybe_key;
+        if (key->Equals(heap->empty_string())) return this;
       }
 
       PropertyDetails details = DetailsAt(i);

test/mjsunit/regress/regress-fast-empty-string.js

+// Copyright 2014 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.
+
+var o = {};
+o[""] = 1;
+var x = {__proto__:o};
+for (i = 0; i < 3; i++) {
+  o[""];
+}
+for (i = 0; i < 3; i++) {
+  assertEquals(undefined, o.x);
+}