function onBundle( error, bundle ) {
		var opts;
		if ( error ) {
			debug( 'Encountered an error when creating a bundle: %s', error.message );
			return cb( error );
		}
		debug( 'Successfully created a bundle.' );
		if ( out === void 0 ) {
			return cb( null, bundle );
		}
		debug( 'Writing bundle to file...' );
		opts = {
			'encoding': 'utf8'
		};
		writeFile( out, bundle, opts, onWrite );
	}
Callback invoked upon creating a bundle.

@private
@param {(Error|null)} error - error object
@param {Buffer} bundle - bundle
@returns {void}