Fix webpackDevMiddleware to work with Koa2, a Promise must always be returned (#177)

* Fix webpackDevMiddleware to always return a Promise in order to be able to await on it for koa flow and always chain with next if required.

* fix(eslintrc): Add es6 env

* Wrap shared.ready method call into a Promise for koa2 support

* Use normal form for an anonymous function not the arrow form

* fix missing closing }

* Remove unexpected space before function parentheses, fix lint

Author: lbdremy

.eslintrc

@@ -1,5 +1,6 @@
 {
     "env": {
+        "es6": true,
         "node": true,
         "mocha": true
     },

middleware.js

@@ -26,10 +26,12 @@ module.exports = function(compiler, options) {
 	function webpackDevMiddleware(req, res, next) {
 		function goNext() {
 			if(!context.options.serverSideRender) return next();
-			shared.ready(function() {
-				res.locals.webpackStats = context.webpackStats;
-				next();
-			}, req);
+			return new Promise(function(resolve) {
+				shared.ready(function() {
+					res.locals.webpackStats = context.webpackStats;
+					resolve(next());
+				}, req);
+			});
 		}
 
 		if(req.method !== "GET") {
@@ -39,39 +41,41 @@ module.exports = function(compiler, options) {
 		var filename = getFilenameFromUrl(context.options.publicPath, context.compiler, req.url);
 		if(filename === false) return goNext();
 
-
-		shared.handleRequest(filename, processRequest, req);
-
-		function processRequest() {
-			try {
-				var stat = context.fs.statSync(filename);
-				if(!stat.isFile()) {
-					if(stat.isDirectory()) {
-						filename = pathJoin(filename, context.options.index || "index.html");
-						stat = context.fs.statSync(filename);
-						if(!stat.isFile()) throw "next";
-					} else {
-						throw "next";
+		return new Promise(function(resolve) {
+			shared.handleRequest(filename, processRequest, req);
+			function processRequest() {
+				try {
+					var stat = context.fs.statSync(filename);
+					if(!stat.isFile()) {
+						if(stat.isDirectory()) {
+							filename = pathJoin(filename, context.options.index || "index.html");
+							stat = context.fs.statSync(filename);
+							if(!stat.isFile()) throw "next";
+						} else {
+							throw "next";
+						}
 					}
+				} catch(e) {
+					return resolve(goNext());
 				}
-			} catch(e) {
-				return goNext();
-			}
 
-			// server content
-			var content = context.fs.readFileSync(filename);
-			content = shared.handleRangeHeaders(content, req, res);
-			res.setHeader("Content-Type", mime.lookup(filename) + "; charset=UTF-8");
-			res.setHeader("Content-Length", content.length);
-			if(context.options.headers) {
-				for(var name in context.options.headers) {
-					res.setHeader(name, context.options.headers[name]);
+				// server content
+				var content = context.fs.readFileSync(filename);
+				content = shared.handleRangeHeaders(content, req, res);
+				res.setHeader("Content-Type", mime.lookup(filename) + "; charset=UTF-8");
+				res.setHeader("Content-Length", content.length);
+				if(context.options.headers) {
+					for(var name in context.options.headers) {
+						res.setHeader(name, context.options.headers[name]);
+					}
 				}
+				// Express automatically sets the statusCode to 200, but not all servers do (Koa).
+				res.statusCode = res.statusCode || 200;
+				if(res.send) res.send(content);
+				else res.end(content);
+				resolve();
 			}
-
-			if(res.send) res.send(content);
-			else res.end(content);
-		}
+		});
 	}
 
 	webpackDevMiddleware.getFilenameFromUrl = getFilenameFromUrl.bind(this, context.options.publicPath, context.compiler);