Added small generator testing framework that allows developers to
create a generator context from synthetic sources outside a GWT
compilation run.

Also moving test/com/google/gwt/dev/javac/impl to
src/com/google/gwt/dev/javac/testing/impl.

Review at http://gwt-code-reviews.appspot.com/1424805


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@10049 8db76d5a-ed1c-0410-87a9-c151d255dfc7