Additional SingleJso fixes for tag interfaces.
Also changes how CastNormalizer selects Cast / instanceof methods for SingleJso types.

Patch by: bobv
Review by: spoon (TBR)
Verified by: macpherson


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@5467 8db76d5a-ed1c-0410-87a9-c151d255dfc7