The toplevel code generator assumed that declarations did not shadow

The toplevel code generator assumed that declarations did not shadow
parameters.  This could case the initial value to be lost or worse, a
crash.

Fix by handling the case of a declaration shadowing both
stack-allocated parameters and those in the arguments object.

This is related to V8 issue 540.
http://code.google.com/p/v8/issues/detail?id=540

BUG=29565
Review URL: http://codereview.chromium.org/469006

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3429 ce2b1a6d-e550-0410-aec6-3dcde31c8c00