code: function(builder, parent, name, options, isGroup, syntax) {
              Namespace.call(this, builder, parent, name, options, syntax);
  
              /**
               * @override
               */
              this.className = "Message";
  
              /**
               * Extensions range.
               * @type {!Array.<number>|undefined}
               * @expose
               */
              this.extensions = undefined;
  
              /**
               * Runtime message class.
               * @type {?function(new:ProtoBuf.Builder.Message)}
               * @expose
               */
              this.clazz = null;
  
              /**
               * Whether this is a legacy group or not.
               * @type {boolean}
               * @expose
               */
              this.isGroup = !!isGroup;
  
              // The following cached collections are used to efficiently iterate over or look up fields when decoding.
  
              /**
               * Cached fields.
               * @type {?Array.<!ProtoBuf.Reflect.Message.Field>}
               * @private
               */
              this._fields = null;
  
              /**
               * Cached fields by id.
               * @type {?Object.<number,!ProtoBuf.Reflect.Message.Field>}
               * @private
               */
              this._fieldsById = null;
  
              /**
               * Cached fields by name.
               * @type {?Object.<string,!ProtoBuf.Reflect.Message.Field>}
               * @private
               */
              this._fieldsByName = null;
          }
docs: Constructs a new Message.
  @exports ProtoBuf.Reflect.Message
  @param {!ProtoBuf.Builder} builder Builder reference
  @param {!ProtoBuf.Reflect.Namespace} parent Parent message or namespace
  @param {string} name Message name
  @param {Object.<string,*>=} options Message options
  @param {boolean=} isGroup `true` if this is a legacy group
  @param {string?} syntax The syntax level of this definition (e.g., proto3)
  @constructor
  @extends ProtoBuf.Reflect.Namespace