Minor cleanup in SerializableTypeOracleBuilder:
- remove unused fields
- TypeInfoComputed can be static
- replace getTypeInfoComputed with ensureTypeInfoComputed or direct access
- tighten visibility
- remove some indirection
- fix some warnings

Review at http://gwt-code-reviews.appspot.com/1869803

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11378 8db76d5a-ed1c-0410-87a9-c151d255dfc7