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.
Exactly.  The compiler should expect the result of calling getView() to have the methods of IView.  Since setRotation() is not a method of IView, the compiler doesn't know that it is a valid method and should complain.  In theory, the result of map.getView() should be assigned to a variable that has some kind of type cast to say that its actually a View2D - this is allowed because View2D "is a" IView - and then calling setRotation() would pass the compiler's static checks.

Basically all the comments that indicate types of things in OpenLayers are all about providing type hints to the compiler (because javascript is dynamically typed) so it can do static analysis (among other things) of the code to ensure maximum correctness.