Set ecmaVersion to 8 for eslint

This allows usage of EcmaScript 2017 and async and await in
particular.

Change-Id: Ib1ab12e87b5c17648a11cac3d935ab86607d88d1
diff --git a/polygerrit-ui/app/.eslintrc.json b/polygerrit-ui/app/.eslintrc.json
index 7cb1a11..97151f2 100644
--- a/polygerrit-ui/app/.eslintrc.json
+++ b/polygerrit-ui/app/.eslintrc.json
@@ -1,5 +1,8 @@
 {
   "extends": ["eslint:recommended", "google"],
+  "parserOptions": {
+    "ecmaVersion": 8
+  },
   "env": {
     "browser": true,
     "es6": true