Added an assertion that style attribute names do not contain a hyphen.  Users should use the camelCase form of the attribute name.

Patch by: jlabanca
Review by: jgw
Issue: 2667

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@3667 8db76d5a-ed1c-0410-87a9-c151d255dfc7