Update css-base.js

Author: e2tox

lib/css-base.js

@@ -68,7 +68,7 @@ function cssWithMappingToString(item, useSourceMap) {
 
 // Adapted from convert-source-map (MIT)
 function toComment(sourceMap) {
-  var base64 = new Buffer(JSON.stringify(sourceMap)).toString('base64');
+  var base64 = window.btoa(JSON.stringify(sourceMap));
   var data = 'sourceMappingURL=data:application/json;charset=utf-8;base64,' + base64;
 
   return '/*# ' + data + ' */';