refactor compute_char_frequency() (#2152)

- minimise maintenance when updating AST
- maximise code sharing between `master` & `harmony`

lib/output.js

@@ -501,6 +501,7 @@ function OutputStream(options) {
             use_asm = prev_use_asm;
         }
     });
+    AST_Node.DEFMETHOD("_print", AST_Node.prototype.print);
 
     AST_Node.DEFMETHOD("print_to_string", function(options){
         var s = OutputStream(options);