function updateExtensions() {
        return Async.doInParallel_aggregateErrors(
            Object.keys(_idsToUpdate),
            function (id) {
                var installationResult = _idsToUpdate[id];
                return update(installationResult.name, installationResult.localPath, installationResult.keepFile);
            }
        );
    }
Updates extensions previously marked for update.
@return {$.Promise} A promise that's resolved when all extensions are updated, or rejected
    if one or more extensions can't be updated. When rejected, the argument will be an
    array of error objects, each of which contains an "item" property with the id of the
    failed extension and an "error" property with the actual error.