Correct issue 696 with Date.parse returning a value when called on a non date string.

Correct issue 696 with Date.parse returning a value when called on a non date string.

The error was introduced in revision 4557 where support was added for
ES5 date time format strings. Because there was no check for a valid
year a random string starting with a non-digit character would be
parsed.

This change disallows ES5 formatted dates where there is no date
fraction (i.e., with only a timestamp). Since none of the other
browsers support Date.parse on only timestamps I have disabled this
totally instead of just correcting the parser.

Review URL: http://codereview.chromium.org/2017005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4613 ce2b1a6d-e550-0410-aec6-3dcde31c8c00