delegate(instantiate = undefined) {
    if (ENABLE_ASSERTS) {
      this.delegatesConstructed_ = true;
    }
    return this.registered_.length ?
        this.instantiate_(this.registered_[0], instantiate) :
        undefined;
  }
Returns the first (highest priority) registered delegate, or undefined
if none was registered.
@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 {T|undefined}