delegates(instantiate = undefined) {
    if (ENABLE_ASSERTS) {
      this.delegatesConstructed_ = true;
    }
    return freeze(this.registered_.map(r => this.instantiate_(r, instantiate)));
  }
Returns an array of all registered delegates, creating a fresh instance
of any registered classes.  The `instantiate` argument can be passed to
override how constructors are called.  The array will be frozen in debug
mode.
@param {(function(function(new: T, ...?)): T)=} instantiate A function to
    instantiate constructors registered with `registerClass`.  By default,
    this just calls the constructor with no arguments.
@return {!Array<T>}