Patch from Steven Keuchel <keuchel@chromium.org>

BUG=v8:2198
LOG=N
TEST=mjsunit/harmony/block-let-crankshaft.js
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/307593002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21684 ce2b1a6d-e550-0410-aec6-3dcde31c8c00