marks a character key as held down while recording a sequence

@param {string} key
@returns void
function _recordKey(key) {
        var i;

        // one-off implementation of Array.indexOf, since IE6-9 don't support it
        for (i = 0; i < _currentRecordedKeys.length; ++i) {
            if (_currentRecordedKeys[i] === key) {
                return;
            }
        }

        _currentRecordedKeys.push(key);

        if (key.length === 1) {
            _recordedCharacterKey = true;
        }
    }