[fix] mkdirp is not called properly on Windows

Author: corporateanon

lib/Compiler.js

@@ -2,6 +2,7 @@
 	MIT License http://www.opensource.org/licenses/mit-license.php
 	Author Tobias Koppers @sokra
 */
+var path = require("path");
 var clone = require("clone");
 var Tapable = require("tapable");
 
@@ -235,9 +236,9 @@ Compiler.prototype.emitAssets = function(compilation, callback) {
 			if(queryStringIdx >= 0) {
 				targetFile = targetFile.substr(0, queryStringIdx);
 			}
-			if(targetFile.indexOf("/") >= 0) {
-				var idx = targetFile.lastIndexOf("/");
-				var dir = targetFile.substr(0, idx);
+
+			if(targetFile.match(/\/|\\/)) {
+				var dir = path.dirname(targetFile);
 				this.outputFileSystem.mkdirp(this.outputFileSystem.join(outputPath, dir), writeOut.bind(this));
 			} else writeOut.call(this);
 			function writeOut(err) {