constructor() {
    /** @private @const {!Array<!Registration<T>>} */
    this.registered_ = [];
    /** @private {boolean} */
    this.allowLateRegistration_ = false;
    /** @private {boolean} */
    this.cacheInstantiation_ = false;
    /** @private {boolean} */
    this.delegatesConstructed_ = false;
  }
Base class for delegate registries.  Does not specify a policy for handling
multiple delegates.
@template T