Merged r14433 into trunk branch.

Get isolate from thread local instead of (potentially missing) block in HConstant.

R=ulan@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/13890014

git-svn-id: https://v8.googlecode.com/svn/trunk@14435 ce2b1a6d-e550-0410-aec6-3dcde31c8c00