Allow runAsync to download code using XHR instead of
script tags.  When the XHR downloads are used, download
errors are detected and downloads can be retried later.

Review by: bobv



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@4917 8db76d5a-ed1c-0410-87a9-c151d255dfc7
5 files changed