Exercise RunAsyncCode class for prefetching fragments.

Recent changes to code splitting fixed a bug in RunAsyncCode.isLoaded().
I wanted to write a test to see that things were really working.

Review at http://gwt-code-reviews.appspot.com/1453805


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10307 8db76d5a-ed1c-0410-87a9-c151d255dfc7