Only write virtual files if there's a webpack instance initialized. Make cssEmit test pass

Author: kaisermann

index.js

@@ -116,7 +116,10 @@ module.exports = function(source, map) {
 			);
 			css.code += '\n/*# sourceMappingURL=' + css.map.toUrl() + '*/'
 			js.code = js.code + `\nrequire('${cssFilepath}');\n`
-			virtualModules.writeModule(cssFilepath, css.code)
+			/** If the webpack compiler is initialized, write the file to its vitual file system */
+			if(this.fs) {
+				virtualModules.writeModule(cssFilepath, css.code)
+			}
 		}
 
 		if (options.hotReload && !isProduction && !isServer) {