Add assert for null provided fields, fixes #7024

Added a test to ensure the assertion happens.

Repost from Rietveld issue 1604803
Thanks to stephenh for the original patch!

Review at http://gwt-code-reviews.appspot.com/1602805


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10783 8db76d5a-ed1c-0410-87a9-c151d255dfc7
5 files changed