Add support for initialization block assignments in the toplevel code

Add support for initialization block assignments in the toplevel code
generator, mimicing the behavior of the optimizing compiler.

Initialization blocks can only contain (thus begin and end) with a
property assignment in toplevel code.

Review URL: http://codereview.chromium.org/348038

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3198 ce2b1a6d-e550-0410-aec6-3dcde31c8c00