AngularJS Error: Cannot use ‘in’ operator to search for ‘$ctrl’ in

I have the directory component passing a function to the question component. The question component is to take an input, and then update the directory component with this input via the function. When the question component tries to access this function, there is the JS error: angular.min.js:123 TypeError: Cannot use 'in' operator to search for '$ctrl' in Lee. What am I doing incorrectly to trigger this error?

https://plnkr.co/edit/hXjhhJcCWPcavp8Z8BRa?p=preview

let directoryTemplate = 
  '<question on-click="$ctrl.updateLastName()"></question>' +
  '<br />' +
  'Full name: {{$ctrl.fullName}}';

class directoryController {
  constructor() {
    this.fullName;
    this.firstName = 'Jack';
  }
  updateLastName(lastName) {
    this.fullName = `${this.firstName} ${lastName}`;
  }
}


let questionTemplate = 
  '<input ng-model="$ctrl.input" />' +
  '<button ng-click="$ctrl.onClick($ctrl.input)">Submit</button>';

class questionController {
  constructor() {
    this.input;
  }
}


angular
  .module('app', [])
  .component('directory', {
    template: directoryTemplate,
    controller: directoryController
  })
  .component('question', {
    template: questionTemplate,
    controller: questionController,
    bindings: {
      onClick: '&'
    }
  });

Source: AngularJS