This is necessary because polymorphic stores generally
do not perform a map check but only an instance type check,
which misses out on changes in the observation status.
Unfortunately, there currently is no efficient way in V8
to maintain that optimisation in the presence of Object.observe.

R=mstarzinger@chromium.org
BUG=v8:2409

Review URL: https://codereview.chromium.org/11477006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13205 ce2b1a6d-e550-0410-aec6-3dcde31c8c00