When AsyncFragmentLoader checks for the success of an XHR download,
it now requires that xhr.responseText is non-null and non-empty.
Otherwise, the download is considered a failure regardless of the
status code.

Review by: bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5267 8db76d5a-ed1c-0410-87a9-c151d255dfc7