Do we still need to set this (in engine_v8.cpp):
setObject( "args", *argsObject, readOnlyArgs); // for backwards compatibility

Dont think this variable is documented anywhere.
Was observed to slow down execution over many invoke (e.g. MR) and also reported by 2 more people to be slow and potentially leak memory.

Right now seems like it's used in only 1 test:
jstests/js7.js:assert.eq( 17 , db.eval( function()

, 17 ) );

Instead the automatice variable 'arguments' can be used.