addKey(keyName, keyEventState) {
    this._setKeyState(keyName, [
      KeyEventStateArrayManager.newRecord(),
      KeyEventStateArrayManager.newRecord(KeyEventType.keydown, keyEventState)
    ]);
  }
Add a new key to the combination (starting with a state of keydown)
@param {ReactKeyName} keyName Name of key
@param {KeyEventState} keyEventState State key is in
@returns {void}