function _classForDocument(doc) {
        if (doc.getLanguage().getId() === "css") {
            return LiveCSSDocument;
        }

        if (LiveDevelopmentUtils.isHtmlFileExt(doc.file.fullPath)) {
            return LiveHTMLDocument;
        }

        return null;
    }
@private
Determine which live document class should be used for a given document
@param {Document} document The document we want to create a live document for.
@return {function} The constructor for the live document class; will be a subclass of LiveDocument.