Lint internal scripts with eslint:recommended (#1729)

* Lint internal scripts with eslint:recommended

* Warnings r bad

packages/react-dev-utils/webpackHotDevClient.js

@@ -7,6 +7,8 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
+'use strict';
+
 // This alternative WebpackDevServer combines the functionality of:
 // https://github.com/webpack/webpack-dev-server/blob/webpack-1/client/index.js
 // https://github.com/webpack/webpack/blob/webpack-1/hot/dev-server.js
@@ -323,4 +325,4 @@ function tryApplyUpdates(onHotUpdateSuccess) {
       }
     );
   }
-};
+}