node: add signature to SET_PROTOTYPE_METHOD

This prevents segfaults when a native method is reassigned to a
different object (which corrupts args.This()).  When unwrapping,
clients should use args.Holder() instead of args.This().

Closes #6690.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
lib: use triple equals

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
domain: don't crash on "throw null"

Signed-off-by: Trevor Norris <trev.norris@gmail.com>