a few changes to make A/B testing easier

common/assets.js

@@ -0,0 +1,32 @@
+const genmap = require('../build/generate_asset_map');
+const isServer = typeof genmap === 'function';
+const prefix = isServer ? '/' : '';
+let manifest = {};
+try {
+  //eslint-disable-next-line node/no-missing-require
+  manifest = require('../dist/manifest.json');
+} catch (e) {
+  // use middleware
+}
+
+const assets = isServer ? manifest : genmap;
+
+function getAsset(name) {
+  return prefix + assets[name];
+}
+
+const instance = {
+  get: getAsset,
+  setMiddleware: function(middleware) {
+    if (middleware) {
+      instance.get = function getAssetWithMiddleware(name) {
+        const f = middleware.fileSystem.readFileSync(
+          middleware.getFilenameFromUrl('/manifest.json')
+        );
+        return prefix + JSON.parse(f)[name];
+      };
+    }
+  }
+};
+
+module.exports = instance;