code: function(builder, message, rule, type, name, id, options) {
              Field.call(this, builder, message, rule, /* keytype = */ null, type, name, id, options);
  
              /**
               * Extension reference.
               * @type {!ProtoBuf.Reflect.Extension}
               * @expose
               */
              this.extension;
          }
docs: Constructs a new Message ExtensionField.
  @exports ProtoBuf.Reflect.Message.ExtensionField
  @param {!ProtoBuf.Builder} builder Builder reference
  @param {!ProtoBuf.Reflect.Message} message Message reference
  @param {string} rule Rule, one of requried, optional, repeated
  @param {string} type Data type, e.g. int32
  @param {string} name Field name
  @param {number} id Unique field id
  @param {!Object.<string,*>=} options Options
  @constructor
  @extends ProtoBuf.Reflect.Message.Field