Change from ECMA version 8 to 9

This allows using object spread syntax.

Change-Id: I644afe32f75d6bbcb197b27f573df452d3995905
diff --git a/polygerrit-ui/app/.eslintrc.js b/polygerrit-ui/app/.eslintrc.js
index b249d78..fc24278 100644
--- a/polygerrit-ui/app/.eslintrc.js
+++ b/polygerrit-ui/app/.eslintrc.js
@@ -21,7 +21,7 @@
 module.exports = {
   "extends": ["eslint:recommended", "google"],
   "parserOptions": {
-    "ecmaVersion": 8,
+    "ecmaVersion": 9,
     "sourceType": "module"
   },
   "env": {