function _markText(editor) {
        var cache = _cachedValues[editor.document.file.fullPath],
            dom = cache && cache.dom;

        if (!dom) {
            console.error("Couldn't find the dom for " + editor.document.file.fullPath);
            return;
        }

        _markTextFromDOM(editor, dom);
    }
Mark the text for the specified editor. Either scanDocument() or
generateInstrumentedHTML() must be called before this function
is called.

NOTE: This function is "private" for now (has a leading underscore), since
the API is likely to change in the future.

@param {Editor} editor The editor whose text should be marked.
@return none