Enforce prod mode, return a Promise so it can be used programmatically

lib/build.js

@@ -1,3 +1,5 @@
+process.env.NODE_ENV = 'production';
+
 const fs = require('fs');
 const path = require('path');
 const mkdirp = require('mkdirp');
@@ -14,15 +16,16 @@ module.exports = () => {
 	// create main.js and server-routes.js
 	create_app();
 
+	return new Promise((fulfil, reject) => {
 		function handleErrors(err, stats) {
 			if (err) {
-			console.error(err ? err.details || err.stack || err.message || err : 'Unknown error');
+				reject(err);
 				process.exit(1);
 			}
 
 			if (stats.hasErrors()) {
-			console.log(stats.toString({ colors: true }));
-			process.exit(1);
+				console.error(stats.toString({ colors: true }));
+				reject(new Error(`Encountered errors while building app`));
 			}
 		}
 
@@ -37,6 +40,8 @@ module.exports = () => {
 				fs.writeFileSync(path.join(dest, 'stats.server.json'), JSON.stringify(serverInfo, null, '  '));
 
 				generate_asset_cache(clientInfo, serverInfo);
+				fulfil();
+			});
 		});
 	});
 };