static createButton() {
    const button = document.createElement('button');
    button.setAttribute('type', 'button');
    return /** @type {!HTMLButtonElement} */ (button);
  }
Create a "button" element with the correct type.

The compiler is very picky about the use of the "disabled" property on
HTMLElement, since it is only defined on certain subclasses of that.  This
method merely creates a button and casts it to the correct type.

@return {!HTMLButtonElement}