8044638: Tidy up Nashorn codebase for code standards
Reviewed-by: lagergren, sundar

8010652: Eliminate non-child references in Block/FunctionNode, and make few node types immutable
Reviewed-by: jlaskey, lagergren

8005843: refSymbols lookup of unbound variable could cause NullPointerException in Lower
Reviewed-by: hannesw, attila