Adding a null check to ShowcaseGenerator so that we don't try to write to a public resource that has already been created.  This fixes a bug when multiple examples reference the same raw source files.

Review at http://gwt-code-reviews.appspot.com/646802

Review by: jgw@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8304 8db76d5a-ed1c-0410-87a9-c151d255dfc7