function getOpenDocumentForPath(fullPath) {
        var id;

        // Need to walk all open documents and check for matching path. We can't
        // use getFileForPath(fullPath).id since the file it returns won't match
        // an Untitled document's InMemoryFile.
        for (id in _openDocuments) {
            if (_openDocuments.hasOwnProperty(id)) {
                if (_openDocuments[id].file.fullPath === fullPath) {
                    return _openDocuments[id];
                }
            }
        }
        return null;
    }
Returns the existing open Document for the given file, or null if the file is not open ('open'
means referenced by the UI somewhere). If you will hang onto the Document, you must addRef()
it; see {@link #getDocumentForPath} for details.
@param {!string} fullPath
@return {?Document}