Add warning to users if it looks like they have run npm start wrongly

We want to catch what we suspect will be a common case, where the user
runs `npm start` having been used to that from before, and then ends up
with a prototype that isn't working and no idea why. In those
circumstances we should give the users a heads-up by outputting
something to the terminal. While we can't be certain of when this might
have happened, we can make a reasonable guess, and the downside of
logging to a hosting services console erroneously is negligible.

bin/cli

@@ -90,9 +90,27 @@ const getChosenKitDependency = () => {
     }
   }
   return defaultValue
-};
+}
+
+// do some heuristics to try and catch situations where a user has run
+// `npm start` (the wrong command) locally and warn them.
+function warnIfNpmStart (argv, env) {
+  if (
+    argv.command === 'start' && // if user ran serve script then assume they know what they want
+    env.NODE_ENV !== 'production' && // some hosting services set NODE_ENV
+    env.PORT === undefined && // some hosting services set PORT
+    env.PASSWORD === undefined // user should have set PASSWORD when setting up hosting
+  ) {
+    console.warn('Warning: It looks like you may have run the command `npm start` locally')
+    console.warn('if you are using the kit on a personal machine you should use `npm run dev`')
+    console.warn()
+    console.warn('If you are seeing this warning in production, it may indicate that your')
+    console.warn('hosting service needs further configuration.')
+    console.warn()
+  }
+}
 
-(async () => {
+;(async () => {
   if (argv.command === 'create') {
     // Install as a two-stage bootstrap process.
     //
@@ -168,6 +186,7 @@ const getChosenKitDependency = () => {
   } else if (argv.command === 'dev') {
     require('../start')
   } else if (argv.command === 'start' || argv.command === 'serve') {
+    warnIfNpmStart(argv, process.env)
     process.env.NODE_ENV = process.env.NODE_ENV || 'production'
     generateAssetsSync()
     require('../listen-on-port')