Depends on the version of GWT, but here’s the latest definition. (Recently all IE9 / IE10 has been removed)
UPDATE: the module has moved from com.google.gwt.user.UserAgent to com.google.gwt.useragent.UserAgent, link above updated.
UPDATE 2: GWT sources have moved to gwt.googlesource.com
Update 3: GWT sources have moved to github.com, link updated.