byId(id) {
    if (isObject(this.entries[id]) && this.entries[id].id === id) {
      const objectId = this.entries[id][OBJECT_ID]
      const path = this.path.concat([{key: id, objectId}])
      return this.context.instantiateObject(path, objectId, ['id'])
    }
  }
Returns a proxied version of the row with ID `id`. This row object can be
modified within a change callback.