code: function(builder, parent, name, field) {
              T.call(this, builder, parent, name);
  
              /**
               * Extended message field.
               * @type {!ProtoBuf.Reflect.Message.Field}
               * @expose
               */
              this.field = field;
          }
docs: An extension (field).
  @exports ProtoBuf.Reflect.Extension
  @constructor
  @param {!ProtoBuf.Builder} builder Builder reference
  @param {!ProtoBuf.Reflect.T} parent Parent object
  @param {string} name Object name
  @param {!ProtoBuf.Reflect.Message.Field} field Extension field