Add api-checker-reference assets to GWT_TOOLS

In preparation for https://gwt-review.googlesource.com/2500



git-svn-id: http://google-web-toolkit.googlecode.com/svn/tools@11618 8db76d5a-ed1c-0410-87a9-c151d255dfc7
add requestfactory-apt.jar from GWT 2.5.1

git-svn-id: http://google-web-toolkit.googlecode.com/svn/tools@11638 8db76d5a-ed1c-0410-87a9-c151d255dfc7
Added rebased guava-15.0


git-svn-id: http://google-web-toolkit.googlecode.com/svn/tools@11639 8db76d5a-ed1c-0410-87a9-c151d255dfc7
Merge branch 'upstream'

Change-Id: Ibbed90d095a27dca0f3f7046895a0de827618ce6