createCancelBtn_() {
    const cancelBtn = document.createElement('button');
    cancelBtn.className = 'blocklyModalBtn';
    cancelBtn.textContent = Blockly.Msg['TYPED_VAR_MODAL_CANCEL_BUTTON'];
    this.addEvent_(cancelBtn, 'click', this, this.onCancel_);
    return cancelBtn;
  }
Create button in charge of cancelling.
@returns {!HTMLButtonElement} The button in charge of cancelling.
@protected