Expose the javascript line number through a java API

When in Java from within a Javascript entry-point, it is sometimes useful to know the javascript line number, for debug purposes for example. For example, if an exception is thrown through java, knowing which javascript line number called the java method is useful for debugging.

A nice interface for this may be simply to maintain a __FILE__ and __LINE__ variables as part of the javascript context. That way java may simply need to extract these variable values in order to report on them.