Check that we only extend reference types.  Avoids a cast error in X10.
Check that we only extend reference types.  Avoids a cast error in X10.

blue-systems-group/project.maybe.polyglot,liujed/polyglot-eclipse,xcv58/polyglot,blue-systems-group/project.maybe.polyglot,blue-systems-group/project.maybe.polyglot,blue-systems-group/project.maybe.polyglot,liujed/polyglot-eclipse,xcv58/polyglot,xcv58/polyglot,liujed/polyglot-eclipse,liujed/polyglot-eclipse,xcv58/polyglot