Add @extends annotations to generated externs
Do not extend goog.* types
Walk up the inheritance chain to an exportable constructor
Add base classes to info.json and generated externs

This ensures that the inheritance chain is intact, even if the base
class is not exportable.
Merge pull request #2644 from ahocevar/extends-externs

Add @extends annotations to generated externs
ol.layer.Vector.getSource return type specialisation.
Remove unnecessary type cast