fix mangle on export (#2133)

- `export default ...`
- `export` with `AST_Destructuring`

fixes #2129

lib/scope.js

@@ -234,8 +234,14 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
         }
 
         function mark_export(def, level) {
+            if (in_destructuring) {
+                var i = 0;
+                do {
+                    level++;
+                } while (tw.parent(i++) !== in_destructuring);
+            }
             var node = tw.parent(level);
-            def.export = node instanceof AST_Export && !node.is_default;
+            def.export = node instanceof AST_Export;
         }
     });
     self.walk(tw);