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

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

Index: test/mjsunit/regress/regress-fast-empty-string.js
diff --git a/test/mjsunit/regress/regress-3159.js b/test/mjsunit/regress/regress-fast-empty-string.js
similarity index 50%
copy from test/mjsunit/regress/regress-3159.js
copy to test/mjsunit/regress/regress-fast-empty-string.js
index cfc8a39b8d011533ed94fcaf11669814c9cbc4d2..9b9fea963a4f7f094283ce5b42289127cb7f594d 100644
--- a/test/mjsunit/regress/regress-3159.js
+++ b/test/mjsunit/regress/regress-fast-empty-string.js
@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-try {
-  new Uint32Array(new ArrayBuffer(1), 2, 3);
-} catch (e) {
-  assertEquals("start offset of Uint32Array should be a multiple of 4",
-               e.message);
+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);
 }