remove Symbol's argument when we're unsafe and it's undeclared

lib/compress.js

@@ -1831,6 +1831,11 @@ merge(Compressor.prototype, {
                         }
                     }
                     break;
+                  case "Symbol":
+                    // Symbol's argument is only used for debugging.
+                    self.args = [];
+                    return self;
+                    break;
                 }
             }
             else if (exp instanceof AST_Dot && exp.property == "toString" && self.args.length == 0) {