The problem was I incorrectly treated NULL result as failure to fetch
a property with a getter.  However, if getter returns zero, it is
manifested as NULL pointer (see added test case).

Good news: that gives another boost as before this CL if getter returned
0, I did another slow lookup.

Review URL: http://codereview.chromium.org/119172

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2106 ce2b1a6d-e550-0410-aec6-3dcde31c8c00