support for API_HOST env variable

README.md

@@ -29,6 +29,18 @@ npm start
 Then go to [http://localhost:8601/](http://localhost:8601/) - the playground outputs the default GUI component
 
 ## Developing alongside other Scratch repositories
+
+### Linking this code to another project's `node_modules/scratch-gui`
+
+#### Configuration
+
+`npm start` can be configured with the following environment variables
+
+| Variable        | Default                            | Description                                    |
+| --------------- | ---------------------------------- | ---------------------------------------------- |
+| `API_HOST`      | `https://api.scratch.mit.edu`      | Hostname for API requests                      |
+
+
 If you wish to develop scratch-gui alongside other scratch repositories that depend on it, you may wish
 to have the other repositories use your local scratch-gui build instead of fetching the current production
 version of the scratch-gui that is found by default using `npm install`.
@@ -43,7 +55,7 @@ To do this:
 
 Instead of `BUILD_MODE=dist npm run build` you can also use `BUILD_MODE=dist npm run watch`, however this may be unreliable.
 
-### Oh no! It didn't work!
+#### Oh no! It didn't work!
 * Follow the recipe above step by step and don't change the order. It is especially important to run npm first because installing after the linking will reset the linking.
 * Make sure the repositories are siblings on your machine's file tree.
 * If you have multiple Terminal tabs or windows open for the different Scratch repositories, make sure to use the same node version in all of them.

webpack.config.js

@@ -130,6 +130,7 @@ module.exports = [
         plugins: base.plugins.concat([
             new webpack.DefinePlugin({
                 'process.env.NODE_ENV': '"' + process.env.NODE_ENV + '"',
+                'process.env.API_HOST': '"' + (process.env.API_HOST || 'https://api.scratch.mit.edu') + '"',
                 'process.env.DEBUG': Boolean(process.env.DEBUG),
                 'process.env.GA_ID': '"' + (process.env.GA_ID || 'UA-000000-01') + '"'
             }),