Adds some assertions to RichTextArea to prevent users from using the formatters before the RichTextArea is initialized.

Patch by: jlabanca
Review by: jgw
Issue: 2749



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5637 8db76d5a-ed1c-0410-87a9-c151d255dfc7