Flatten cons string for single character substrings.

For substrings of non-flat cons strings, we bail out
to runtime. For single character substrings, we forget
to flatten it. This causes successive bailouts.

R=bmeurer@chromium.org
BUG=323041
LOG=Y

Review URL: https://codereview.chromium.org/88173002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18081 ce2b1a6d-e550-0410-aec6-3dcde31c8c00