Use HtmlUnit in Graal-JS

[vermgit]

After reading #755 I'm wondering if it would be possible (and maybe easier) to do the other way around: as Graal-JS seems to be able to handle latest JS-stuff, but has no way to work like a full-fledged browser with DOM-support, maybe HtmlUnit can be loaded into Graal-JS somehow and those two exchange the internal stuff as needed?

[rbri]

This is a ongoing topic, and at the moment Rhino moves forward and add support for more and more features.

The problem with changing the js engine is the DOM stuff. All classes marked with @JsxClass are more or less js engine specific adapters that have to be migrated when changing the engine. At the moment we have ~600 such classes.