Fix performance regression caused by r11202.

test/mjsunit/regress/regress-2156.js

Index: test/mjsunit/regress/regress-2156.js
diff --git a/src/platform-posix.h b/test/mjsunit/regress/regress-2156.js
similarity index 81%
copy from src/platform-posix.h
copy to test/mjsunit/regress/regress-2156.js
index 7a982ed2ef3080dad77860d6f46b356a9067bf3b..348257113062b568770a072d86ed52b3e854d384 100644
--- a/src/platform-posix.h
+++ b/test/mjsunit/regress/regress-2156.js
@@ -25,15 +25,15 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#ifndef V8_PLATFORM_POSIX_H_
-#define V8_PLATFORM_POSIX_H_
+// Flags: --allow-natives-syntax --harmony-collections
 
-namespace v8 {
-namespace internal {
+var key1 = {};
+var key2 = {};
+var map = new WeakMap;
 
-// Used by platform implementation files during OS::PostSetUp().
-void POSIXPostSetUp();
-
-} }  // namespace v8::internal
-
-#endif  // V8_PLATFORM_POSIX_H_
+// Adding hidden properties preserves map sharing. Putting the key into
+// a WeakMap will cause the first hidden property to be added.
+assertTrue(%HaveSameMap(key1, key2));
+map.set(key1, 1);
+map.set(key2, 2);
+assertTrue(%HaveSameMap(key1, key2));