* Made AST IDs constant. Making this explicit helps understanding the code.

* Removed a few dead functions.

* Be honest by making node_type() pure virtual.

All these changes have been checked to be performance-neutral.

Review URL: https://chromiumcodereview.appspot.com/10796058

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00