Avoid dynamic lookup when initializing let declared variables.

Avoid dynamic lookup when initializing let declared variables.

'Let's inside a 'with' would initialize the variable
using the StoreContextSlot runtime function which
would fail because it checks that the variable does
not hold the hole value.

Committed: http://code.google.com/p/v8/source/detail?r=9156

[Steven, 2011-09-02 14:05:15]

PTAL.

[fschneider, 2011-09-05 11:41:05]

LGTM.