Cache ModuleDefs by context ClassLoader.

GWT Designer need separated class loader for every open project.

http://gwt-code-reviews.appspot.com/1274801/show
Patch by: amitin
Review by: me,zundel


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@9538 8db76d5a-ed1c-0410-87a9-c151d255dfc7