build(aio): add staging environment

You can now specify what environment you are building
by add it to the `yarn build` command. For example:

```
yarn build -- --env=stage
```

Moreover the `deploy-to-firebase.sh` script will automatically apply the
appropriate environment.

aio/scripts/deploy-to-firebase.sh

@@ -8,11 +8,13 @@ readonly deployEnv=$1
 
 case $deployEnv in
   staging)
+    readonly buildEnv=stage
     readonly projectId=aio-staging
     readonly deployedUrl=https://$projectId.firebaseapp.com/
     readonly firebaseToken=$FIREBASE_TOKEN
     ;;
   production)
+    readonly buildEnv=prod
     readonly projectId=angular-io
     readonly deployedUrl=https://angular.io/
     readonly firebaseToken=$FIREBASE_TOKEN
@@ -27,7 +29,7 @@ esac
   cd "`dirname $0`/.."
 
   # Build the app
-  yarn build
+  yarn build -- --env=$buildEnv
 
   # Deploy to Firebase
   firebase use "$projectId" --token "$firebaseToken"