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.
The compiler should see it as returning an `ol.IView` though, which does not have `setRotation` and so compiling that example in advanced mode should trigger some kind of warning or error.