UnifyAst correctly handles polymorphic overrides with mixed default/public access.

GWT AST now tracks method access.  This is used by UnifyAst to correctly compute overrides.  Before, UnifyAst would allow a public method to override a default-access method in a different package, which is illegal.
http://gwt-code-reviews.appspot.com/1470803/


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10422 8db76d5a-ed1c-0410-87a9-c151d255dfc7