Make the native check support Rhino. Close gh-185.

In Rhino, native functions do not display as just [native code],
but often with extra information, for example:

   js> Array.prototype.slice
   function slice() { [native code for Array.slice, arity=2] }
Fix the slice check. Close gh-184.

As of 9751a47ded9693140af2aa635f7a9d928ac30a0b, docElem is not defined
at the point of its use in the slice check, so it always fails, and the
native slice never gets used.