reactive(tracker) {
    if (tracker && Object.keys(this._deps).length === 0) {
      this._depsAny = new tracker.Dependency();
      this._schemaKeys.forEach((key) => {
        this._deps[key] = new tracker.Dependency();
      });
    }
  }
Makes this validation context
reactive for Meteor-Tracker.
@param tracker {Tracker}