onKey_(event) {
    const keyCodes = goog.events.KeyCodes;
    if (event.keyCode == keyCodes.SPACE || event.keyCode == keyCodes.ENTER) {
      this.changeColor_();
    }
  }
Fired when user presses a key while the DIV has focus. If the user presses
space or enter, the color will be changed.
@param {goog.events.Event} event The key event.
@private
@suppress {strictMissingProperties} missing 'keyCode' prop