Updates the reflective visitor pattern uses in GenerateJavaAST
to cache Method instance lookups.  This cuts the runtime
of GenerateJavaAST.exec() in half.

Review at http://gwt-code-reviews.appspot.com/817802


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8683 8db76d5a-ed1c-0410-87a9-c151d255dfc7