Throw if assigning to read only element.
Adding tests for element assignment in strict mode.

Fix tests for strict mode SetElement.

Review URL: http://codereview.chromium.org/6613005/

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7053 ce2b1a6d-e550-0410-aec6-3dcde31c8c00