Do not use generic StringBuilderConcat which requires array passed
to keep both elements and separator (which roughly double size
of the array).  That should be faster as well.

BUG=crbug.com/54580

Review URL: http://codereview.chromium.org/6520004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6777 ce2b1a6d-e550-0410-aec6-3dcde31c8c00