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.
ok, so you're saying it should be:
```js
    var view = /** @type {ol.View2D} */ (this_.getMap().getView());
    view.setRotation(0);
```
I just tried this with warning_level:verbose and it doesn't make any difference to the messages.

https://github.com/google/closure-compiler/wiki/Experimental-Type-Based-Property-Renaming does not mention interfaces, but does say that the compiler 'degrades gracefully' in the absence of full type info. I take that to mean that this is not an error, just that it's not able to rename optimally.

I can add this type cast if you like. It would be consistent with the type casts in other examples.