Configure webpack-dev-server (#5786)

* setup devserver with correct proxy

* dev server wroking with written manifest

* add comments

* Change webpack to default port 9000 and minor js formatting

* Use hash in development.

* write to disk = true

* Take ports as argument for dev-server

* update instructions

* update instructions

* add devtools

* use mode instead of NODE_ENV

* use minicssextract for prod (like before)

* remove empty line

CONTRIBUTING.md

@@ -261,23 +261,52 @@ To parse and generate bundled files for superset, run either of the
 following commands. The `dev` flag will keep the npm script running and
 re-run it upon any changes within the assets directory.
 
-```
+```bash
 # Copies a conf file from the frontend to the backend
 npm run sync-backend
 
 # Compiles the production / optimized js & css
 npm run prod
 
+# Start a watcher that rebundle your assets as you modify them
+npm run dev
+
 # Start a web server that manages and updates your assets as you modify them
-npm run dev
+npm run dev-server
 ```
 
-For every development session you will have to start a flask dev server
-as well as an npm watcher
+For every development session you will have to
 
-```
+1. Start a flask dev server
+
+```bash
+superset runserver -d
+# or specify port
 superset runserver -d -p 8081
-npm run dev
+```
+
+2. Start webpack dev server
+
+```bash
+npm run dev-server
+```
+
+This will start `webpack-dev-server` at port 9000 and you can access Superset at localhost:9000.
+By default, `webpack-dev-server` is configured for flask running at port 8088.
+
+If you start flask server at another port (e.g. 8081), you have to pass an extra argument
+`supersetPort` to `webpack-dev-server`
+
+```bash
+npm run dev-server -- --supersetPort=8081
+```
+
+You can also specify port for `webpack-dev-server`
+
+```bash
+npm run dev-server -- --port=9001
+# or with both dev-server port and superset port
+npm run dev-server -- --port=9001 --supersetPort=8081
 ```
 
 #### Upgrading npm packages