Mark AbstractNode as abstract class

[bitPogo]

According to the dec block in the types as well as in the code AbstractNode is a abstract BaseClass. For more semantical expression it should be marked as a abstract class in the type definition.

[ggrossetie]

Indeed, even though you cannot really extend AbstractNode or at least that's not the right way to do it since AbstractNode is a Ruby class. So if you want to extend AbstractNode you will need to create a new class with AbstractNode as a superclass using Opal.allocate_class (from Opal runtime):

https://github.com/opal/opal/blob/505fd5d7e899a6766472b302c079b4d383fd8216/opal/corelib/runtime.js#L452

[bitPogo]

Thats true! Maybe this could be compensated by adding @sealed.
Also this applies for most the classes (maybe even for all of them) in this project.