Shell extensions no longer load due to fix for bug 646333:

    let metadataContents;
    try {
     >>>   metadataContents = Shell.get_file_contents_utf8_sync(metadataFile.get_path());
    } catch (e) {
        global.logError(baseErrorString + 'Failed to load metadata.json: ' + e);
        return;
    }

Error displayed by Looking Glass is:

   "ReferenceError: Shell is not defined"
 
Fix is to add following line to top of file:

   const Shell = imports.gi.Shell;