Allow objects with "" properties to stay fast.

test/mjsunit/test-hidden-string.js

Index: test/mjsunit/test-hidden-string.js
diff --git a/test/mjsunit/regress/regress-347542.js b/test/mjsunit/test-hidden-string.js
similarity index 58%
copy from test/mjsunit/regress/regress-347542.js
copy to test/mjsunit/test-hidden-string.js
index 901d798fb7fbea45f0d9f3d8ba6c7a9846bf6dd6..a5d32c839ea38e08abedc2d95542009accdec36c 100644
--- a/test/mjsunit/regress/regress-347542.js
+++ b/test/mjsunit/test-hidden-string.js
@@ -4,8 +4,8 @@
 
 // Flags: --allow-natives-syntax
 
-function foo() {}
-foo();
-%OptimizeFunctionOnNextCall(foo);
-foo();
-%NeverOptimizeFunction(foo);
+var o = {};
+%SetHiddenProperty(o, "test", 1);
+// Create non-internalized ""
+var empty = "a".substring(1, 1);
+assertEquals(undefined, o[empty]);