init() {
    this._super(...arguments);

    this._externalRoutes = {};
  }
A cache of dependency names and values, grouped by engine name.

    This cache is maintained by `buildChildEngineInstance()` for every engine
    that's a child of this parent instance.

    Only dependencies that are singletons are currently allowed, which makes
    this safe.

    @private
    @property {Object} _dependenciesForChildEngines