Don't export View2D.getView2D

Small tidy-up. See https://groups.google.com/forum/#!topic/ol3-dev/wdyZgN2GuY4

Not sure whether the function is needed at all, but it is currently used in several places in the lib, so it can keep for the moment.
my knowledge of how the compiler handles interfaces is limited, but AIUI an interface simply means that anything implementing that interface must include the methods in that interface; it doesn't prevent it from having other methods that aren't in the interface. View2D implements IView2D which doesn't have calculateExtent