Exports fixes

The commit messages should be pretty explicit.
 	 
With 1f641c5 this is what the generated Export constructors (in `build/src/external/src/exports.js`) now look like:

```
/**
 * @constructor
 * @extends {ol.Map}
 * @param {olx.MapOptionsExtern} options Options.
 */
ol.MapExport = function(options) {
  /** @type {ol.MapOptions} */
  var arg;
  if (goog.isDefAndNotNull(options)) {
    arg = {
      controls: options.controls,
      interactions: options.interactions,
      layers: options.layers,
      renderer: options.renderer,
      renderers: options.renderers,
      target: options.target,
      view: options.view
    };
  } else {
    arg = /** @type {ol.MapOptions} */ (options);
  }
  goog.base(this, arg);
};
```