Retain comments before AST_Constants during mangle.

lib/output.js

@@ -409,6 +409,7 @@ function OutputStream(options) {
 
     AST_Node.DEFMETHOD("print_to_string", function(options){
         var s = OutputStream(options);
+        if (!options) s._readonly = true;
         this.print(s);
         return s.get();
     });
@@ -416,6 +417,7 @@ function OutputStream(options) {
     /* -----[ comments ]----- */
 
     AST_Node.DEFMETHOD("add_comments", function(output){
+        if (output._readonly) return;
         var c = output.option("comments"), self = this;
         var start = self.start;
         if (start && !start._comments_dumped) {