Support GWT.runAsync by splitting the JavaScript code
into independently downloadable program fragments.

This commit is a merge from /changes/spoon/runAsync,
but with many small cleanups.

Review by: bobv


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@3901 8db76d5a-ed1c-0410-87a9-c151d255dfc7