Makes Document.createUniqueId() return guaranteed document-unique id's (using a counter on $doc).

Issue: 2376
Patch by: jgw
Review by: jlabanca


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@2699 8db76d5a-ed1c-0410-87a9-c151d255dfc7