UC16 representation we need to be careful about flat strings.
Flat strings can be sliced or cons strings that have a flat
string under them, so when we ask a flat cons or a slice whether
it is ASCII or not we should ask the underlying string about
its representation.  This should fix
http://code.google.com/p/chromium/issues/detail?id=10971
Review URL: http://codereview.chromium.org/100249

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1830 ce2b1a6d-e550-0410-aec6-3dcde31c8c00