fix(serve): fix gitignore, fix lint problems

Author: knagaitsev

packages/serve/.gitignore

@@ -1 +1,3 @@
 *.js.map
+*.js
+!flags.js

packages/serve/argsToCamelCase.ts

@@ -19,7 +19,7 @@ function dashesToCamelCase(str): string {
  * @returns {Object} - the same args object as passed in, with new keys
  */
 export default function argsToCamelCase(args): object {
-    Object.keys(args).forEach((key) => {
+    Object.keys(args).forEach((key): void => {
         const newKey = dashesToCamelCase(key);
         if (key !== newKey) {
             const arg = args[key];

packages/serve/startDevServer.ts

@@ -12,7 +12,7 @@ import * as Server from "webpack-dev-server/lib/Server";
  */
 export default function startDevServer(compiler, options, onListening): void {
     const server = new Server(compiler, options);
-    server.listen(options.port, options.host, (err) => {
+    server.listen(options.port, options.host, (err): void => {
         if (err) {
             throw err;
         }